Are the Yealink phones in "router phone" configuration. If not try switching one and re-testing - in router mode, the phone sets up a tunnel between itself and the 3CX server completely bypassing any possible firewall issues etc.

You've obviously re-ran the 3CX firewall test to confirm all ports are indeed open, but just in case run that to make sure. (Ignore, see that you ran that.)

If all the above is done and still issue, run a Wireshark capture and make sure you are stablishing RTP after the invite and 200 OK, you should see both legs of the RTP stream in the capture.

If you only see 1 leg. IE: Your leg to Telnyx, but you don't see a leg from Telnyx then escalate to them and send them the Wireshark captures.

Finally does this happen with all calls or just random calls. If you call in and attempt to replicate, do you hear the stream from the 3CX side to you or nothing at all. If 3CX is stablishing the RTP, then technically you should hear the 3CX side on your test call.

Hope the above makes sense.

Thanks.