how do user agents interact with 3cx

[regfixit]

My set up is SPA3102 connected to PSTN and its Line 1 is registered as 3cx extension; Thomson ST780(WL) phone 1 and phone 2 also registered as 3cx extensions - these 2 devices sit on the same LAN as my PC that is running 3cx server.
Remotely I have a Thomson ST780(WL) that has phone 1 registered as a 3cx extension to my PC.
VOIP gateway via voiptalk.org.

I was wondering how the calls actually get routed so I can set up things for best call quality. I was thinking that since my SPA3102 and home ST780 are on the LAN then bandwidth is no issue and can use G711 codec and set RTP size to 10 ms in their config settings. However, I was wondering how outbound calls get handled - does the 3cx sit in the middle and relay the conversion or does it set up a conversation directly between the devices.

Eg. I dial out from local extenstion on the ST780, the outbound rule in 3cx says use voiptalk. Does it mean:

a) My ST780 has a conversation with the 3cx over the LAN which in turn is talking to the voiptalk server over the internet
or
b) My ST780 gets redirected to talk directly to the voiptalk server over the internet

Does the "PBX delivers audio" setting make any difference to how the flow of data works ?

 

[archie]

In the case of "PBX delivers audio" your audio (RTP stream) will be handled by 3CX Media Server. It means your phone will send/receive RTP to/from MS, and MS will send/receive RTP to/from VoIP provider. Signalling is always going thru 3CX SIP server.

 

[regfixit]

I just found the technical manual posted by Archie in another thread. This is exactly what I was looking for. Extremely useful.