Mobile Phone Numbers/ Cell Phones

I am desperate for some help on this one. The problem is now accepting numbers starting with zero but it does involve formatting/ masks.

I have a sheet with many mobile/ cell phone numbers:
07771123456 for example
But I need to change this format to drop the "0" and add the country code "44", so need to automate this somehow with a mask/macro.

07771123456 needs to change to 447771123456 and there is thousand of these so I cant face any more hours of doing it manually, there is a way isnt there? please for my sanity

I use office 2007.