Michigan, United States
Eastern Time
10:41 AM
Area Code
231
State / Province
Michigan
Country
United States
Time Zone
Eastern Time
Area code 231 covers 99 cities and communities across Michigan, based on telephone company rate center data.
Use the E.164 format for clickable phone links (RFC 3966):
<a href="tel:+12315550100">+12315550100</a><a href="tel:+12315550199">+12315550199</a>Look up which exchange in Michigan a specific 231 number is homed to, and whether its block is wireline or wireless. See what a reverse lookup can establish before relying on the answer.
572 active prefixes in this area code, assigned as 3,773 thousands-blocks — pooling splits one prefix across several carriers, so each block is listed separately with the rate centre it is homed to, its carrier of record, and the switch that serves it.
Showing blocks 1–100 of 3,773
| NPA-NXX | Block | Rate centre | Type | Carrier (OCN) | Switch | LATA | LIR | Dates |
|---|---|---|---|---|---|---|---|---|
| 231-201 | 0 | Chippewa Lake | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-201 | 1 | Chippewa Lake | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-201 | 2 | Chippewa Lake | Landline or VoIP | BANDWIDTH.COM CLEC, LLC - MI (991E) | GDRPMIBL00Z | 348 | — | — |
| 231-201 | 3 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | 4 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | 5 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | 6 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | 7 | Chippewa Lake | Landline or VoIP | CENTURYLINK COMMUNICATIONS, LLC (508J) | TRCYMIMNWMD | 348 | — | — |
| 231-201 | 8 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | 9 | Chippewa Lake | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-201 | A | Chippewa Lake | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | 0 | Mecosta | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-202 | 1 | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | 2 | Mecosta | Landline or VoIP | BANDWIDTH.COM CLEC, LLC - MI (991E) | GDRPMIBL00Z | 348 | — | — |
| 231-202 | 3 | Mecosta | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-202 | 4 | Mecosta | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-202 | 5 | Mecosta | Wireless | ONVOY SPECTRUM, LLC (624H) | GDRPMIBL36Z | 348 | — | — |
| 231-202 | 6 | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | 7 | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | 8 | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | 9 | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-202 | A | Mecosta | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 0 | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 1 | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 2 | Alanson | Landline or VoIP | CHARTER IP ENABLED SERVICES, LLC (792K) | GFTPMIEF2MD | 348 | — | — |
| 231-203 | 3 | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 4 | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 5 | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-203 | 6 | Alanson | Landline or VoIP | LEVEL 3 COMMUNICATIONS, LLC - MI (6120) | GDRPMIQLX5Y | 348 | — | — |
| 231-203 | 7 | Alanson | Landline or VoIP | CENTURYLINK COMMUNICATIONS, LLC (508J) | TRCYMIMNWMD | 348 | — | — |
| 231-203 | 8 | Alanson | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-203 | 9 | Alanson | Landline or VoIP | BANDWIDTH.COM CLEC, LLC - MI (991E) | GDRPMIBL00Z | 348 | — | — |
| 231-203 | A | Alanson | Wireless | NEW CINGULAR WIRELESS PCS, LLC - IL (6534) | WYNGMIKAGT2 | 348 | — | — |
| 231-204 | A | Fife Lake | Landline or VoIP | SPOK, INC. (6630) | GDRPMIBL44Z | 348 | — | — |
| 231-205 | A | Brutus | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-206 | 0 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 1 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 2 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 3 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 4 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 5 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 6 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 7 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 8 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | 9 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-206 | A | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-207 | A | Honor | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-208 | A | Boyne Falls | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-209 | A | Suttons Bay | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-210 | 0 | St James | Landline or VoIP | TERRA NOVA TELECOM INC. (145J) | — | 348 | — | — |
| 231-210 | 2 | St James | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-210 | 6 | St James | Landline or VoIP | TELEPORT COMMUNICATIONS AMERICA, LLC - MI (770E) | GDRPMIBL06Z | 348 | — | — |
| 231-210 | A | St James | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-212 | 0 | Interlochen | Landline or VoIP | 123.NET, LLC (2550) | GDRPMIBLX8Z | 348 | — | — |
| 231-212 | 1 | Interlochen | Wireless | ONVOY SPECTRUM, LLC (624H) | GDRPMIBL36Z | 348 | — | — |
| 231-212 | 3 | Interlochen | Wireless | ONVOY SPECTRUM, LLC (624H) | GDRPMIBL36Z | 348 | — | — |
| 231-212 | 5 | Interlochen | Wireless | ONVOY SPECTRUM, LLC (624H) | GDRPMIBL36Z | 348 | — | — |
| 231-212 | 7 | Interlochen | Landline or VoIP | CENTURYLINK COMMUNICATIONS, LLC (508J) | TRCYMIMNWMD | 348 | — | — |
| 231-212 | 9 | Interlochen | Landline or VoIP | PEERLESS NETWORK OF MICHIGAN, LLC - MI (933G) | BERRIEN SPRINGS | 348 | — | — |
| 231-212 | A | Interlochen | Landline or VoIP | CENTURYLINK COMMUNICATIONS, LLC (508J) | TRCYMIMNWMD | 348 | — | — |
| 231-213 | 0 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 1 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 2 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 3 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 4 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 5 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 6 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 7 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 8 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | 9 | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-213 | A | Traverse City | Landline | MICHIGAN BELL TEL CO (9323) | TRAVERSE CITY | 348 | — | — |
| 231-214 | 1 | Lakes of the North | Landline or VoIP | CHARTER IP ENABLED SERVICES, LLC (792K) | ANTSMIAD0MD | 348 | — | — |
| 231-214 | A | Lakes of the North | Landline or VoIP | CHARTER IP ENABLED SERVICES, LLC (792K) | ANTSMIAD0MD | 348 | — | — |
| 231-215 | A | Muskegon | Wireless | CELLCO PARTNERSHIP DBA VERIZON WIRELESS - MI (6003) | GDRQMIBODMD | 348 | — | — |
| 231-216 | 0 | Frankfort | Landline or VoIP | BANDWIDTH.COM CLEC, LLC - MI (991E) | GDRPMIBL00Z | 348 | — | — |
| 231-216 | 1 | Frankfort | Landline or VoIP | PEERLESS NETWORK OF MICHIGAN, LLC - MI (933G) | BERRIEN SPRINGS | 348 | — | — |
| 231-216 | 2 | Frankfort | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-216 | 3 | Frankfort | Landline or VoIP | VONAGE AMERICA LLC (197D) | BRSPMI092MD | 348 | — | — |
| 231-216 | 4 | Frankfort | Landline or VoIP | MCIMETRO ACCESS TRANSMISSION SERVICES LLC - MI (7157) | GRAND RAPIDS | 348 | — | — |
| 231-216 | 5 | Frankfort | Landline or VoIP | PEERLESS NETWORK OF MICHIGAN, LLC - MI (933G) | BERRIEN SPRINGS | 348 | — | — |
| 231-216 | 6 | Frankfort | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-216 | 7 | Frankfort | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-216 | 8 | Frankfort | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-216 | 9 | Frankfort | Landline or VoIP | ONVOY, LLCV - MI (543C) | GDRPMIBL04Z | 348 | — | — |
| 231-216 | A | Frankfort | Landline or VoIP | PEERLESS NETWORK OF MICHIGAN, LLC - MI (933G) | BERRIEN SPRINGS | 348 | — | — |
| 231-217 | A | Bear Lake | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-218 | 0 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 1 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 2 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 3 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 4 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 5 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 6 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 7 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 8 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | 9 | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-218 | A | Traverse City | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
| 231-219 | A | Merritt | Landline or VoIP | TELNET WORLDWIDE, INC. (817J) | GDRPMIBQ1MD | 348 | — | — |
| 231-220 | 0 | Muskegon | Landline or VoIP | 123.NET, LLC (2550) | GDRPMIBLX8Z | 348 | — | — |
| 231-220 | 1 | Muskegon | Wireless | OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC (2964) | GDRPMIBL31Z | 348 | — | — |
Block-level assignments from the North American Numbering Plan administrator, via localcallingguide.com.
Assigned Prefixes
572
Assignment Blocks
3,773
Rate Centers
99
Carriers of Record
53
| Type | Blocks | Share |
|---|---|---|
| Landline | 664 | 18% |
| Landline or VoIP | 2,031 | 54% |
| Wireless | 1,078 | 29% |
| Carrier | Blocks |
|---|---|
| ONVOY, LLCV - MI | 744 |
| NEW CINGULAR WIRELESS PCS, LLC - IL | 378 |
| MICHIGAN BELL TEL CO | 305 |
| CELLCO PARTNERSHIP DBA VERIZON WIRELESS - MI | 285 |
| OMNIPOINT COMMUNICATIONS MIDWEST OPERATIONS, LLC | 283 |
| FRONTIER NORTH, INC. - MI | 236 |
| BANDWIDTH.COM CLEC, LLC - MI | 162 |
| LEVEL 3 COMMUNICATIONS, LLC - MI | 156 |
| CHARTER IP ENABLED SERVICES, LLC | 143 |
| ONVOY SPECTRUM, LLC | 115 |
| 43 other carriers | 966 |
Carrier of record for each block. Numbers ported to another carrier keep their original block assignment, so this is not a lookup of who serves a given number today.
| Rate Center | Prefixes | Blocks |
|---|---|---|
| Muskegon | 68 | 521 |
| Traverse City | 53 | 284 |
| Petoskey | 22 | 131 |
| Cadillac | 21 | 133 |
| Big Rapids | 19 | 121 |
| Ludington | 14 | 71 |
| Cheboygan | 13 | 89 |
| Manistee | 12 | 89 |
| Kalkaska | 10 | 63 |
| Fremont | 9 | 64 |
| Newaygo | 9 | 67 |
| Boyne City | 8 | 50 |
| Beulah | 7 | 49 |
| Charlevoix | 7 | 39 |
| Frankfort | 7 | 47 |
| Harbor Springs | 7 | 49 |
| Hart | 7 | 55 |
| Northport | 7 | 38 |
| Reed City | 7 | 48 |
| Evart | 6 | 31 |
| Fife Lake | 6 | 31 |
| Glen Lake | 6 | 17 |
| McBain | 6 | 26 |
| Morley | 6 | 37 |
| Baldwin | 5 | 34 |
| Bellaire | 5 | 47 |
| Coral | 5 | 32 |
| East Jordan | 5 | 40 |
| Elmira | 5 | 31 |
| Grant | 5 | 40 |
| Mackinaw City | 5 | 31 |
| Mancelona | 5 | 31 |
| Scottville | 5 | 35 |
| Whitehall | 5 | 43 |
| Wolverine | 5 | 30 |
| Alanson | 4 | 23 |
| Alden | 4 | 25 |
| Central Lake | 4 | 22 |
| Fruitport | 4 | 30 |
| Honor | 4 | 21 |
| Howard City | 4 | 28 |
| Lake Ann | 4 | 16 |
| Lake City | 4 | 8 |
| Mecosta | 4 | 25 |
| Moorestown | 4 | 12 |
| Onekama | 4 | 29 |
| Pentwater | 4 | 31 |
| Shelby | 4 | 33 |
| White Cloud | 4 | 30 |
| Williamsburg | 4 | 34 |
| Bear Lake | 3 | 13 |
| Boyne Falls | 3 | 21 |
| Brutus | 3 | 21 |
| Eastport | 3 | 24 |
| Elk Rapids | 3 | 28 |
| Falmouth | 3 | 26 |
| Fountain | 3 | 26 |
| Freesoil | 3 | 21 |
| Harrietta | 3 | 26 |
| Hesperia | 3 | 23 |
| Holton | 3 | 23 |
| Indian River | 3 | 33 |
| Interlochen | 3 | 28 |
| Irons | 3 | 29 |
| Kingsley | 3 | 32 |
| Lake Leelanau | 3 | 32 |
| Le Roy | 3 | 24 |
| Levering | 3 | 13 |
| Luther | 3 | 18 |
| Manton | 3 | 29 |
| Marion | 3 | 26 |
| Merritt | 3 | 16 |
| Ravenna | 3 | 23 |
| Suttons Bay | 3 | 13 |
| Tustin | 3 | 27 |
| Twin Lake | 3 | 25 |
| Walloon Lake | 3 | 24 |
| Alba | 2 | 3 |
| Amble | 2 | 4 |
| Brethren | 2 | 4 |
| Buckley | 2 | 3 |
| Carr | 2 | 15 |
| Chippewa Lake | 2 | 21 |
| Clam River | 2 | 18 |
| Dublin | 2 | 3 |
| Ellsworth | 2 | 15 |
| Kaleva | 2 | 4 |
| Lakes of the North | 2 | 3 |
| Mesick | 2 | 16 |
| Pellston | 2 | 21 |
| St James | 2 | 15 |
| Stanwood | 2 | 18 |
| Torch River Bridge | 2 | 19 |
| Wellston | 2 | 5 |
| Bois Blanc | 1 | 11 |
| Copemish Thompsonville | 1 | 1 |
| Hoxeyville | 1 | 7 |
| Old Mission | 1 | 11 |
| South Boardman | 1 | 1 |
A rate center is the telephone exchange area a number is homed to. It sets local calling boundaries, and is not the caller's current location.
Area code 231 is a North American Numbering Plan (NANP) telephone area code assigned to Michigan, United States. It is one of 13 area codes serving Michigan. As population and demand for phone numbers grew, the region was divided into multiple numbering plan areas to avoid exhausting the available 7-digit number pool.
Yes. 231 is an active North American Numbering Plan (NANP) area code, currently assigned to Michigan, United States. It is safe to answer or dial calls using this area code.
Area code 231 covers 99 cities and communities in Michigan, including Alanson, Alba, Alden, Amble, Baldwin, and 94 more localities. This is based on telephone company rate center data — the geographic areas carriers use to determine local calling boundaries and to assign numbers when a business or individual requests a phone number in a specific city.
All numbers in the 231 area code observe Eastern Time (IANA timezone: America/New_York). Eastern Time is UTC−5 in winter (EST) and UTC−4 in summer (EDT). When placing a call to a 231 number from a different time zone, check the current local time above to avoid calling outside of business hours.
From within Michigan, dial the 7-digit local number directly if calling within the same rate center, or dial 1 + 231 + 7-digit number for long-distance calls within North America. From outside the US or Canada, dial your country's international exit code followed by 1 231 XXX XXXX. The country code for both the United States and Canada in NANP is +1.
Format 231 numbers using E.164 notation as required by RFC 3966: prefix with +1 and omit dashes and parentheses (e.g., +12315550100). This format ensures tel: links work correctly on all mobile browsers and VoIP apps. See the tel: URI formatting guide for complete examples.