Area code 231

At a glance

Country code

+1

State

MI

Exchanges on record

596

Carriers on record

32

Most exchanges: AT&T 154, Frontier 52, Onvoy 49.

The exchange map

Formats and patterns

Any 231 number, in E.164

^\+1231[2-9]\d{6}$

+1, the area code, a central office code that does not start with 0 or 1, then four digits.

^\+1231(?:2(?:0[1-46]|1[58]|3[39]|45|5[03-5]|6[08]|7[48]|8[124-8]|9[09])|3(?:0[17]|1[3578]|2[179]|3[05-79]|4[0-39]|5[07]|60|7[035]|8[03478]|9[02-47])|4(?:0[489]|1[0346]|2[059]|3[2-47]|45|5[046]|6[35689]|9[2-579])|5(?:1[02469]|31|5[027]|6[349]|7[08]|80|9[0368])|6(?:0[458]|1[35]|2[049]|3[1-38]|4[0259]|5[15]|67|7[0569]|90)|7(?:[02]9|1[245]|3[025]|4[02]|5[08]|94)|8(?:18|3[3578]|[46]6|5[25]|7[128]|8[13467])|9(?:07|[14]2|2[03]|39|55|70))\d{4}$

^\+1231(?:2(?:0[57-9]|1[3479]|2[0235-9]|3[024-8]|4[0-3]|5[6-9]|6[1-79]|7[1569]|9[12568])|3(?:0[35689]|1[2469]|2[24-68]|3[1-4]|4[46-8]|5[1-689]|6[1-35689]|7[278]|8[169]|9[589])|4(?:01|1[57]|2[14]|3[69]|4[0189]|5[379]|6[24]|77|8[046-9]|98)|5(?:08|1[58]|2[025-79]|3[3-9]|4[4-9]|6[78]|7[17]|8[1-578]|9[1279])|6(?:14|2[25-7]|3[4-79]|4[36-8]|5[26]|6[08]|7[248]|8[0-3569]|95)|7(?:19|2[0-8]|3[134679]|4[3-57]|5[3579]|6[02-46-9]|7[3-9]|8[028]|9[0-26-9])|8(?:0[3568]|13|2[1-689]|3[02469]|4[2-58]|5[3469]|6[124579]|7[34679]|8[259]|9[03489])|9(?:0[238]|16|2[24589]|3[0235-8]|4[13467]|7[24]|81|9[45]))\d{4}$

^\+1231(?:2(?:1[026]|2[14]|4[4679]|5[12]|7[0237]|8[039]|9[347])|3(?:0[024]|10|2[03]|38|[48]5|6[47]|7[1469]|9[16])|4(?:0[0235-7]|1[289]|2[236-8]|3[0158]|4[2-467]|5[125]|60|75)|5(?:00|59|66|94)|6(?:00|6[36]|88|99)|7(?:07|46|51|7[12]|8[67]|93)|8(?:00|19|31|75|8[08])|9(?:00|6[368]|77|99))\d{4}$

The exchange patterns follow the original allocations; a number ported since keeps its digits but not necessarily its carrier or its line type.

Carriers and their exchanges

Every carrier holding exchanges in 231, with the exchanges allocated to it (the three digits after the area code).

Questions developers ask

Where is area code 231?

Area code 231 is in Michigan, in the United States: all 596 of its exchanges on record are there. From abroad it is dialled +1 231, then the seven-digit number.

Is a 231 number mobile or landline?

Either: in the North American Numbering Plan the line type belongs to the exchange (the three digits after the area code), not to the area code. Of the 596 exchanges on record in 231, 172 (29%) were allocated as mobile, 305 (51%) as fixed line, 95 (16%) as VoIP and 24 (4%) have no line type on record. A number can have been ported since, to another carrier and even to another line type.

Which carriers have numbers in area code 231?

32 carriers hold exchanges in 231 on record; the most: AT&T (154), Frontier (52) and Onvoy (49). These are the original allocations: with number portability, a number may be served by another carrier today, which only a live carrier lookup (an HLR or MNP query) can tell.

How do I validate a 231 phone number?

In E.164 a 231 number matches ^\+1231[2-9]\d{6}$: +1, the area code, a central office code that does not start with 0 or 1, then four digits. Written nationally it reads (231) 555-0123. The exchange patterns on this page narrow it to the exchanges allocated as mobile, fixed line or VoIP.