UC520 calls destination via SIP from AA, directly,via blast grou
My first guess would be that the SIP provider is rejecting the call because the caller ID of the incoming PSTN call is being passed to them. They see this, and do not recognize the number, hence they drop the call. If this is the case, you will probably need to add a translation rule to modify the outgoing caller-id for all outgoing calls to a DID that the SIP provider will accept.
But to be sure about what is happening, we would need to see some debugs of the call. I would start with:
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