How Ofcom allocates UK phone numbers

Every UK phone number — landline, mobile, freephone, premium-rate, personal-numbering — is allocated by Ofcom under the National Telephone Numbering Plan. It's a small bureaucratic miracle that makes UK reverse-lookup possible at all. Here's how the system works in practice.

#Block sizes

Ofcom allocates numbers in 1,000-block chunks (10,000-blocks for some legacy ranges). A 'block' has a fixed area code, then a 1,000-number subscriber range — for example 020-7946-0000 to 020-7946-9999. The block is the unit of allocation, the unit of porting, and the unit of return-to-Ofcom.

#Allocation status

#The weekly numbering data feed

Every Wednesday, Ofcom publishes a ZIP containing all of the above as CSVs — one CSV per area / prefix. Inside each row: the start and end numbers, the Range Holder, the status, and any annotations (e.g. 'Allocated for Numbering Plan Renumbering').

WhoCalledLookup ingests this feed and atomically swaps the live database, keeping per-number queries accurate to within seven days. We documented the ingest internally in scripts/ingest-ofcom.ts if you're curious.

SABC Code,Range,SubRange,Communications Provider,Status
020,7946,0,BT,Reserved (Drama)
020,7946,1,BT,Reserved (Drama)
020,7946,9,BT,Reserved (Drama)

#Drama numbers

Numbers like 020 7946 XXXX, 0114 496 XXXX, 0117 496 XXXX, 07700 900XXX and 0306 999 XXXX are reserved for film, TV and journalism so plot phone numbers don't ring real customers. If you see these in a script, that's why. If you see one calling you in real life, that's a spoofed CLI.

#Porting — why current-carrier ≠ Range Holder

Once a number is in service, the customer can ask their new provider to port it from the old one. The new provider takes over the underlying line; the old provider releases it. Ofcom's records still show the *originally allocated* Range Holder, because that's what the Numbering Plan tracks.

There is no public 'current carrier' feed for UK numbers. The closest available signal is the carrier presented on caller ID, which can itself be spoofed. We unpacked the implications in Range Holder vs current provider.

#What this means for reverse lookups

When you paste a number into our lookup form, we run a longest-prefix match against the ingested Ofcom data. That tells us:

• The Range Holder (originally allocated provider).
• The status (Allocated / Reserved / Free / Protected / Recovered).
• The block size and the start/end of the parent range.
• The area code and (for 01/02) the principal town.

What it doesn't tell us: who actually owns the number today, or what they're using it for. That's where the AI internet check fills in the gap.

#Bottom line

The Ofcom Numbering Data is the single most authoritative free dataset for UK reverse lookup. It's accurate, it's public, it's refreshed weekly — and it's the foundation every UK number-validation vendor builds on. The interesting questions ('is this number a scammer?') need behavioural data on top, but you should always start with what Ofcom says about the line.