Local numbers
The 0203 Area Code, Explained
Straight answer: 0203 is London - all of London. The area code is 020, and local numbers beginning with 3 were opened by Ofcom in 2005 when the older 020 7 and 020 8 ranges ran low. Not a separate code, not a specific district, not a scam range.
ONSIM provides London numbers on your mobile from £5/month + VAT - no app, no VoIP, no desk phone.
Where 0203 came from
When London consolidated on the 020 code in 2000, its numbers lived in two ranges: 020 7 (the old inner-London 0171) and 020 8 (the old outer-London 0181). Demand ate through them, so in 2005 Ofcom opened local numbers beginning with 3 - a fresh batch of millions, allocated London-wide with no district meaning. For the next decade and a half, most new London numbers came from this range.
So a number written 0203 xxx xxxx is really (020) 3xxx xxxx. Same code, same 8-digit local format, same call cost as every other London number - just a younger batch than 7 or 8, and an older one than 4.
Why am I getting calls from 0203?
Sheer volume. Twenty years of new London businesses, call centres, clinics and agencies were numbered in this range, so it dominates call logs in a way 020 7 never will again. People trust the ranges they grew up seeing, and 0203 still reads as unfamiliar to anyone who left London before 2005.
Spam callers use the range too - they use every range, and any number can be spoofed. Judge the call, not the prefix: an unexpected caller wanting bank details, passwords or remote access is a scam whatever number they show. Decode any UK number with the free ONSIM UK phone number checker.
020 3, 020 4, 020 7, 020 8 - all London
Four ranges, one city, one area code.
020 7
Historically inner London (the old 0171). Long established and now scarce for new allocations. About 0207.
020 8
Historically outer London (the old 0181). Also long established and in short supply. About 0208.
020 3
Added in 2005, allocated London-wide with no district meaning. The main source of new London numbers for over a decade - this page.
020 4
Added in 2019, allocated London-wide. The current source of fresh London numbers. About 0204.
Full history of London numbering on the London 020 numbers page.
Getting a London number for your business
After two decades of allocation the 020 3 range is largely spoken for, so a new London number today most likely starts 020 4 - equally London in every way. If a 020 3 number specifically matters to you, ONSIM can discuss availability.
ONSIM routes calls to your London number at network level, not through an app. Start with Virtual Landline and the number rings on your existing mobile; upgrade to Landline SIM when you want outbound calls showing it too.
Pricing
Virtual Landline (from £5/mo + VAT): a London number that rings on your existing mobile. Includes up to 2,500 forwarding minutes. Inbound calls only.
Landline SIM (from £10/user/mo + VAT): inbound and outbound calls showing your London number, delivered natively via SIM/eSIM. Unlimited UK calls under fair use.
0203 number FAQs
Common questions about the 0203 range and London numbering.