Being able to change the default extension

Being able to change the default prefix for extensions, right now, extensions are based of what numerical prefix was initially chosen, IE if your first extension was 605999, subsequint extensions are 60XXXX with no ability to change the prefix.

We would like the ability to change the default prefix, and it would be extra nice if it could be based off of Location code. IE US users are set to prefix 20XXXX, Canada is set to 30XXXX, etc.