The number is often given in the format +447088913819. The country code, (the +44) outwardly indicates the number is UK phone number but it is the 70 prefix which identifies the phone number as being a personal forwarding number.
Many phone providers offer this service for free and the phone number can be "forwarded" to any phone in the world.
The cost for the call, which usually is very high, is billed to the caller, not the receiver of the phone call therefore scammers love this free, portable number as it deceives the victim into thinking the scammer is in the UK,