This utility application will go through your contacts in the address book and transform all local phone numbers (20733445566 or 13108883333) into correct internationalized numbers (+4420733445566, +13108883333).

If your address book is full of local numbers it might not properly identify incoming callers and show their names, instead only showing the incoming number.

This app gives you precise control over what you change and who you change it for: range of digits to match, prefix, etc...

Note: You should always back up and sync your iOS device before making any such major changes to your contacts.