[andy]

I don't quite understand what you mean or want. If you are in one country and want to call another, and if you forward one phone, you will still need another phone to call the first phone, and that call will also cost something. If you use callback to the second (roaming) then the first (diverted) phone, it may or may not all be worthwhile, but whatever you do except a simple call there will be two to four tariffs involved.