No audio whenever SRTP is enforced, even on the *777 echo test.

[PacketPilgrim]

Hi all, looking for help or a second opinion.


Setup: SMB Free tenant hosted by 3CX. Version shown in the console is 20.0 Update 10 (Build 1716 - AI 1.6.63). Yealink T48S desk phones, all on firmware 66.86.0.180, behind a 3CX SBC running on a local Windows PC. SIP over UDP.


What happened: Everything worked fine on 10/06/2026. Overnight it stopped, and I haven’t knowingly changed anything on the 3CX side. Calls connect and ring, but there’s no audio in either direction whenever SRTP is enforced (extensions set to enforce, phones set to Compulsory). Turning SRTP off restores audio right away.


What I’ve found so far:

• It affects every phone, and internal calls too, so it isn’t the trunk.
• Calling *777 from a desk phone with SRTP on gives no audio. The report shows about packets received from the PBX, zero sent from my end, and “No valid RTCP report” and “No audio/RTCP”.
• The *777 test from the 3CX Web Client fails the same way (“Network problems between PBX and extension”), so it doesn’t seem specific to the phones or the SBC.
• The SBC shows as connected. Phone clocks are correct.
• Before I realized how widespread it was, I rebuilt the SBC, factory reset the phones, and manually flashed the 3CX .rom firmware. None of it changed the behavior.

What I’ve read: The change log lists my build as an Update 10 RC from October. I found a few threads about SRTP audio failures after Update 9 that were reportedly fixed in Update 10, but those mostly involve SIP trunks, so I’m not sure they match my case. I don’t see an SRTP change in the RC notes.

Questions:

1. Is there a recent update that likely caused this?
2. Is there any current solution that doesn't involve turning off SRTP?