Free tools
Free UK telecoms tools
Data-backed tools we built while working on our own telephony products. All free, no sign-up, no data captured about your lookups. Underlying data comes from Ofcom, Openreach and other public sources, used under the Open Government Licence.
Number lookup
UK Phone Number Checker
Who called me? What type of number is this?
Paste any UK phone number. See what type of number it is (landline, mobile, freephone, premium), what area it is from, and which network operator was originally allocated the block. All formatted variants (national, international, E.164). Uses Ofcom's official numbering data.
Source: Ofcom Numbering Data + Ofcom Area Codes Tool
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Switch-off planning
PSTN Switch-Off Checker
Am I affected by the January 2027 switch-off?
Answer 5 short questions about your phone, broadband and equipment. Get a personalised verdict on whether the PSTN switch-off affects you, what specifically stops working, and ranked replacement options with honest trade-offs.
Source: Openreach + industry sources
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Compliance
Call Recording Retention Calculator
How long must I keep call recordings?
3 questions. Get the minimum retention period across UK regulatory regimes (FCA SYSC 10A, MiFID II, PCI DSS, MLR 2017, SRA, ICOBS, UK GDPR, FOS). Every rule cited to its source. Built for compliance officers.
Source: FCA Handbook + UK legislation + PCI SSC
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Why we build these
ONSIM is a UK telephony provider running since 2013. We build tools like these while working through the same questions our customers ask us: is this number a scam? What area code will I get if I move? Does the PSTN switch-off affect me? The public data is out there but it is hard to use.
Each tool uses real underlying data (Ofcom's official numbering plan, Openreach's rollout, our own MVNO experience) rather than heuristics or LLM-generated content. Where we do not have the data to answer confidently, we say so.
If you spot a mistake, please let us know. If a tool would help you and we do not have it, tell us that too.
Looking for a paid ONSIM product?
Virtual Landline (£5/mo), a UK landline number that rings your mobile. Landline SIM (from £10/user/mo), a mobile SIM with a landline number. Landline Replacement (from £10/mo), a copper-PSTN drop-in for the 2027 switch-off. See all products on the homepage.