Hello
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 10.1.169.13 on the D735’s but this issue has existed back when we had 10.1.84.15. (10.1.84.12 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.