We have a new sip service but have not ported our numbers to the new provider yet. Surprisingly, it seems possible to make an outbound call using the new service with a non-ported number. Do providers allow calls from numbers that don't belong to them? I didn't think this was possible, but my co-workers was able to do it.
Depends of the provider and what they allow. We have Enterprise Trunking enabled with our provider so we can send any outbound calling number we choose, regardless of whether we own the number or if it lives on the circuit. Sounds your provider allows something similar. It is possible they are only allowing calls from numbers that you, as a customer, own perhaps regardless of the circuit. Best thing would be grab some debugs from the router with the new circuit for a successful call and check them. #1 confirm the call is going out the new circuit and see what external number mask is being sent. I would also modify an external number mask to a number you do not own and see if that is allowed or rejected.