Federal rules say your phone number belongs to you, not your carrier. Any carrier has to release it when a valid port request comes in. The whole fight is in the word valid.
A port gets rejected when the paperwork does not exactly match the losing carrier's records: business name, service address, account number. Fix is simple, pull a copy of your latest phone bill and match it character for character.
The one fatal mistake is canceling old service before the port completes. A dead account has no number to release. Whatever you do, port first, cancel after.
Done right, the cutover takes seconds and callers never notice. We file and track ports every week, and porting is free with any of our service.