+1 231 area code (Michigan): Muskegon, Traverse City, Petoskey

The range at a glance

+1 231 in NANPA’s area code report

+1 231 is a North American area code for Michigan, in service since 5 June 1999, per NANPA’s NPA report of 21 October 2026. NANPA names 616 as the code it came from, split off to serve part of its area.

Rate centres with the most allocated codes (NANPA)

Each central office code is assigned for a rate centre, the regulator’s billing area for it, named after a town or exchange. It is not where a subscriber is: mobile and VoIP numbers are used anywhere, and a ported number keeps its rate centre. The 572 assigned codes in 231 are spread over 99 rate centres.

Codes assigned in the last 12 months, and codes not yet in service

NANPA’s central office code file of 21 October 2026 lists 1 code in 231 assigned in the 12 months to 21 October 2026, and 34 thousands-blocks inside its pooled codes. Not yet in service: 1 thousands-block. NANPA dates codes from 2001 on only: 281 of the 572 codes carry a date, so an undated code is not a new one. A code not yet in service is waiting for its effective date; a reassigned block can hold numbers used before. Original allocation, not who serves a number today.

Blocks in the regulator's table

Each block is a 6-digit prefix of the national number. A number in an allocated block is assignable; in an unassigned block no carrier can have a subscriber, so the lookup answers unassigned_block for it, except a number in a range reserved for fiction (is_fictional), which answers valid even where the regulator lists its block as protected. Of the allocated blocks, 25.5% belong to a wireless carrier, 14.5% to a virtual-number wholesaler or VoIP provider, and 60.0% to a holder whose name settles neither.

Who the blocks were allocated to

The holders as the regulator lists them, by number of allocated blocks. Allocation is not service: a ported number keeps its original block, so a holder here may no longer serve a given number. The last row adds up every other allocated block in the table: the smaller holders and any holder this page does not name. 382 of these codes are pooled into thousands-blocks, which carriers other than the code holder can hold. This page counts codes; a lookup of a US number names the holder of the number's own thousands-block where NANPA lists it.

What each network brand’s legal entities hold across the whole table, counted in numbers: AT&T, Inteliquent, Frontier, Verizon, T-Mobile, Lumen, Charter, Bandwidth, Brightspeed, Spok.

All 572 assigned central office codes in 231

Each row is a central office code (the three digits after the area code) as NANPA’s central office code file lists it: the rate centre it is assigned for, the network brand of the code holder where the brand map knows one, and the date it was assigned where the file gives one. Original allocation, not who serves a number today: a ported number keeps its code, and in a pooled code the blocks of a thousand can be held by carriers other than the code holder. There is no page per code.

How to read this page

Everything above comes from three public sources: the numbering metadata, which names the zones and the line types of the range and, outside North America, its place; and the regulator's allocation table, which lists each block's status and holder and, in North America, the rate centres the place is named from. NANPA’s NPA report adds what it says about the whole area code: its state or province, the codes sharing its area, when it went into service, local dialling and relief planning. None of them knows who serves a number today, whether a handset is switched on, or who owns or answers it. Spaw reports what the tables say and names the dataset and version behind every answer.

More area codes in the United States

• +1 225
• +1 227
• +1 228
• +1 229
• +1 234
• +1 235
• +1 239
• +1 240
• All area codes in Michigan
• All area codes in the United States
• New and upcoming North American area codes
• Look up any number

nanpa-co-codes 2026-10-19 · nanpa-rate-centres 2026-10-18 · nanpa-npa-report 2026-10-21 · newest data dated 2026-10-21 · about the regulator’s table