Many phone handsets now let you see the number of the person calling before you answer.
This feature – known as ‘Caller ID’ or ‘Calling Line Identity’ (CLI) – is a handy way of screening the calls you want to answer from the ones you don’t.
However, there have been growing instances of nuisance callers and criminals deliberately changing the Caller ID, a practice known as ‘spoofing’.
Don’t trust the number displayed.
http://consumers.ofcom.org.uk/phone/tackling-nuisance-calls-and-messages/phone-spoof-scam/