The provider is Skyetel, and they will not/can not re-write the outbound caller ID based on a calling plan/route. My first choice would be to have Skyetel re-write all outbound caller IDs to be the main number UNLESS the dialed number was "911".
That is not the job of the provider. That is the job of the PBX.
FWIW - there are providers that do this (not us though). Its extremely expensive as you have to own all the phone numbers you are using, and the rates to do this are very high.