Final SIP Ack Being Dropped

[User314342]

We have been experiencing an intermittent issue with one of our IP phones since we moved 3CX to Azure, resulting in one-way audio. This is only intermittent, often a redial from the caller will resolve the issue temporarily. Some details about our setup

3CX in Azure
Phone on premises
Site to Site VPN between them (no NATing)
Phone is Yealink T58V
Incoming external calls trigger this issue

After packet capturing on the phone and server simultaneously, I came up with the following packet captures, included below.

The key difference is that the final SIP ACK from sever to phone is never delivered. I checked and this is going to the correct IP with port 5060->5060, just like the previous packets, I believe ruling out firewall issues. Additionally, previous captures seem to indicate it is always this packet this is not arriving. This causes the client phone to sit there waiting for the ACK and never responding to the voice traffic (or sending any of its own).

Does anyone have any idea why this might be happening? Thank you!

Server:



Phone:

 

[ChrisC_3CX]

The fact that you see the ACK leaving the 3CX PBX but never reaches the IP Phone, suggests that the issue may reside on a network node in between the two endpoints.

Something that seems odd though is the length of the SIP ACK message seen on the server side capture. The fact that it is only 35 bytes may mean that the SIP ACK message is getting fragmented, and you can actually see that Wireshark did detect some fragmented packets. You might want to look into that before proceeding any further.