Voice routing with local PBX and remote SBC

[P.B.]

Scenario:

- Local PBX is connected to a SIP Trunk.
- Remote office has a SBC and is connected to local PBX

Question:
It is not clear to me what the SBC does in relation to voice routing for external calls. If a user in the remote office (connected to the SBC) places an external call, how is the voice routed?

Is it: User -> SBC -> PBX -> Trunk
or: User -> SBC -> Trunk (in which the SBC effectively routes the voice directly to the trunk and therefore avoiding any latency on the path SBC->PBX)

 

[ChrisC_3CX]

Is it: User -> SBC -> PBX -> Trunk

It is actually this. The 3CX SBC essentially establishes a Tunnel connection with the 3CX PBX and then routes all SIP Signaling and RTP from the IP Phones to the PBX via that Tunnel connection.

The only scenario which may differ a bit is if you have an internal call between two IP Phones that are behind the same SBC and whose extensions do not have "PBX delivers audio" enabled and are also not recording calls. In this scenario, SIP Signaling will still go to the SBC >> PBX and back, but the RTP traffic will be sent directly from one IP Phone to the other.