When setting up a SIP trunk for a customer there IP PBX or Switch may need to know what number or digits we are sending on the SIP trunk.
This answer depends on where the IP PBX or Switch is looking in the SIP Voice Packet. We treat SIP trunks in the same way as a PBX extension. We will send that extension (SIP trunk) as a SIP URI. For example, the SIP URI would look something like "2000@trk.packetvoice.us" assuming we set the sip trunk up as number 2000. When a DID is assigned to this SIP trunk, we send the full 11 digit DID number in the SIP TO Header. For example, if the DID was 5553334444, we would put in the SIP TO Header "15553334444".
Most IP PBX and Switches have the ability to choose whether to look at the SIP URI or the SIP TO Header.