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We have approximately 200 D735 phones running on 3CX 18.8. All work well except for one issue: there is only one-way audio after a hold or transfer on our local network. We have not tested transfer on the SIP trunks, but we rarely use transfer there. 

We currently have the 3CX supported firmware on the D735’s but this issue has existed back when we had ( did apparently have an issue with one-way audio during transfers according to a 3CX post).

Nothing in our firewall or DNS has changed (we host 3CX on premise, not cloud). I have tried different codecs, our current codec is G722 but we I have tried G711u with the same results: there is only one way audio after coming back from a hold or if a transfer is attempted.

We appreciate any advice on this issue, thank you.


Steve G.

End user

Joined: 26.01.2023


  1. Hi, please try to change the RTP audio stream destination. If I remember correctly in 3CX v18 you can select if the audio is directly from a phone to the other one or pass always through the PBX. In these cases, the second option is preferred.

  2. End user Steve G.

    Thanks for that answer. But how do I change the RTP audio stream destination so it passes through the PBX instead of direct? Is there a downside to that, like too much load on the PBX?

  3. Hi, I'm sorry, but I don't remember exactly where is the option, maybe in the extension page on an advanced menu.

  4. End user Steve G.

    No problem. I did find this option in the 3CX troubleshooting section on a per extension basis: PBX Delivers Audio. 3CX forums confirm checking this will indeed cause audio to always go through the PBX. 3CX says about checking that option: "one of the main downsides is that it utilizes bandwidth and latency on the VoIP call". I will set up a test and try it though.

    We have set up encrypted voice traffic on the LAN. All the D735 audio streams are encrypted. Could that be the cause of the problem?

  5. End user Steve G.

    I have set 'PBX Delivers Audio' for 3 D735 extensions as a test. This forces the audio stream to go through the PBX. We then tried to transfer, then went back to the original call. There was only one-way audio, so the problem was not solved. We have now reverted the 'PBX Delivers Audio' setting in 3CX.

    We appreciate any further suggestions, thanks.