Will all numbers in your table get the same country dialing code added or will you need to look up that code from a table if the user doesn't enter the code manually?
There are a number of posts here that discusss auto-enter calculations that automatically format the phone number and supply missing area codes and such so it should be possible to adapt one of those calculations to meet your needs.
if user doesn't enter the code by manually
Then is this a case where it needs to be added to the phone number in all cases or only when missing?
Will the added code be the same in all cases or should it be looked up from a table by using a field that identifies the country?
will be the same code for all numbers as i have done by calculation value
And will some fields already include that value or is it missing from all your phone numbers?
If this is imported data, an auto-enter calculation can add this data during the import. You can also use Replace Field Contents with the calculation option to update this data after it is imported.
I'm using "1234" as the place holder for the country code that you want to add to the phone number in these examples.
If the data is always missing, it can be as simple as this:
"1234" & Self
as an auto-enter calc or
"1234" & YourTable::Field
if it's done with Replace FIeld Contents.
And you can use an If function to check the left most characters of this number to see if it has already been added and only add it if it is missing:
If ( Left ( self ; 4 ) = "1234" ; self ; "1234" & Self )