if people have a number in their phone in national format it will be hard to tell if mobile or not but if people enter mobile numbers in their address book with +44 7 etc prefix and none mobile numbers as say 01… 02… 03…
The issues we face are someone could have a number in their phone starting +44 7 or 07… but it may be a mobile or it may be a personal number and there is no way to know for sure without reference to a separate database
Also someone may have a number in their contacts next to a UK address starting 07… which is a mobile but another address in their contacts outside the UK with a 07… number but it is not a mobile but a landline in their national format (so dials OK when visiting that country) rather than entered in the international + format.
Therefore you can’t base any number filtering on location but would need to somehow filter the number against the country field of the address (assuming they have bothered to enter it).
That is why it is all complex and would help if users could enter their phone numbers differently for landline and mobile, e.g. national format v international format.
One final issue is if you are relying on a user’s contact list correctly having the number associated with a field called Mobile rather than Home or Office or Other this still may not work as they may have multiple mobiles and hence have different mobiles associated against Home or Office rather than Mobile.