Who Called Me from 01923 (Watford)?

01923 is the UK dialling code for Watford, in the historic county of Hertfordshire, covering an area with a population of approximately 96,768. Numbers in this area follow the format 019231234567 — the dialling code, followed by a local number assigned to a specific Communications Provider (CP).

Ofcom has allocated 302 number blocks within the 01923 range, spread across BT, Virgin Media, Sky, TalkTalk, Gamma and many smaller UK CPs. Each block of 1,000 (or sometimes 10,000) numbers is owned by a single CP, which may then assign individual numbers to its own customers or to resellers further down the chain. You can see the full block-by-block allocation list below.

Although 01923 originally indicated the geographic location of the line, UK number portability means a phone with this prefix can technically be located anywhere. For most landlines the geographic match still holds, but for VoIP-only numbers it does not. If you receive a call from a 01923 number and you don't recognise it, you can match the next four digits (the "block") against the table below to identify the originally-allocated network operator.