many providers ( I can speak first hand for Century Link in iowa) do not allow spoofing of caller id
this is essentially what you end up doing
it just rejects the call if the callerid doesn't match the credentials...
I had this with CenturyLink integrated access, and ended up talking with an engineer that told me that they cant do it, so I asked about their sip trunks and was told it is the same system with the same limits.
I hope you get further than I did, but that is what I was told.
Dave