Thank you for the tips, unfortunately these calls are to my mobile phone, I don't have a land-line. When these calls come in they are from mobile numbers registered as far apart as: Aberdeen, Cardiff, Brighton and all points in between. I've tried blocking them, but they seem to have access to an awful lot of phones and either Virgin Media hire a lot of telemarketers with strong Indian accents and English names, or this is a huge scam. It must be working on some people for them to keep it going on such a large scale.
If the calls were actually coming from VM, and you have no connection with VM, you can still register your mobile no. with TPS to stop unwanted marketing from legitimate companies.
Not sure how you are matching geographical locations to incoming mobile numbers but if other random area codes are seemingly coming from all areas of the country, and claiming to be VM, then these would be scam calls. The caller ID you see is likely to be faked (either it does not exist or may have been stolen from an innocent party and used as a fake caller ID)
It is possible to get mobile phone apps which can identify scam calls (generally based on large databases of known scam numbers). No experience of these myself so don't know how well they work. Some mobiles allow you to silence calls from unknown numbers and direct to voicemail.
Simplest thing to do is if you don't know the number coming in, don't answer immediately. Let it go to voicemail.
Scam callers rely heavily on automated dialling to place their calls. If you keep answering every time they call, you number will be identified as responsive so they will keep dialling you.