What could this be?

I have a system with SIP trunks. I have about 13 DID numbers. I have several companies using the system. We have found that if a user dials the DID of another company within the same system there is no audio. If we dial the Ring Groups that we have setup the call goes thru. Outside calls to the companies appear to go through just fine. Any idea why this is happening??

Yeah, you have not set up a loop around trunk and your DID provider is too stupid to loop audio on their gateways.

Thanks for pointing that out to.