I don't know enough about SIP yet, so apologies if this is a naive question.

If you have a voip-in number from one of these Finarea companies, and it is pre-diverted to another number, then would it be possible to make the callback first leg via Sipbroker to that number, then the ongoing call using Voipcheap, Voipstunt or whichever one you have? Or would the call forward be counted as logged in using the account anyway, and prevent or be knocked out by the second call, as already discussed?