David Musgrave explains how to Changing the Phone Number Format to allow more characters to be inputted through the GP user interface.
Using eConnect it is possible to set the full 21 characters of the phone number when changing or creating a record. However as the default for the UK version of GP only allows fourteen characters keyable length, this results in that phone number being impossible to edit.
Updating the keyable length of the Phone, Phone3, STR14_FAX and STR14_PHONE solved this allowing the full number to be accessible in the UI.
Beware though, this is a modification that will need checking after each GP upgrade as in a recent upgrade we encountered the keyable length falling back to the original, something that was not caught by UAT. It was quickly resolved but would have been nice to have anticipated the issue.