Include automatic phone number -> gateway translation.

It would be nice (if unnecessary) for the gateway to be automatically found and used for any give number.

Additionally, a flag for overriding would be needed.

The issue does not name any files, tests, or entry points. First locate how phone numbers and email-to-SMS gateways are selected, then clarify the automatic lookup and override flag behavior. Done means a number can select its gateway automatically while an explicit override remains available.

Written by the indexing model from the issue text.