Solved Yealink SIP-T46S and SIP-T41S dropped calls behind SBC

[MarkSitzler]

Hi all,

We are currently experiencing an issue with Yealink phones (SIP-T46S and SIP-T41S; all on firmware 66.86.0.180) that randomly drop calls. When the calls drop, all live calls are dropped.
They are connected to an SBC (latest version) on a Windows Server 2024 located locally.
Our 3CX version is Version 20.0 Update 7 (Build 1080 Release), and hosted externally from the site.
They are passing through a FortiGate firewall with SIP ALG disabled.
On the SBC, we have been seeing errors similar to the ones below (unclear whether these are related).
ERR | 20260131-180337.219 | 3CX | SBC | 0x2758 | Dns.cpp:80 | [DNS] Failed to retrieve SRV records for _3cxtunnel._tcp.(our_3cxHostname); errcode=12
ERR | 20260131-180407.410 | 3CX | SBC | 0x2758 | TunnelTcp.cpp:305 | Bridge failure on while processing TCP: Connection timeout

 

[JohnS_3CX]

Hi,

In this case you can ignore the phones, the evidence you shared points away from those and towards some kind of network issues.

Essentially focus on the fact that your SBC connection is dropping and why. You already see two red flags: DNS, and connection timeouts. This leans more towards the SBC site host machine, network and internet connection being suspect.

 

[MarkSitzler]

Hi,

In this case you can ignore the phones, the evidence you shared points away from those and towards some kind of network issues.

Essentially focus on the fact that your SBC connection is dropping and why. You already see two red flags: DNS, and connection timeouts. This leans more towards the SBC site host machine, network and internet connection being suspect.

That was our assumption as well. We will remove the SBC and connect the phones via the FQDN to see if that helps.
Thank you for your input, @JohnS_3CX!

 

[JohnS_3CX]

We will remove the SBC and connect the phones via the FQDN to see if that helps.

Actually that will make things worse. You will have the same network issues as before, and you will be adding NAT and ALG issues on top of that.

Stick to what we know and try to figure out what is making the SBC connection drop. That's your best bet for success.

 

[MarkSitzler]

Found out it was the client's backend internet provider, and the DNS was not set correctly! Thank you for your help!

Actually that will make things worse. You will have the same network issues as before, and you will be adding NAT and ALG issues on top of that.

Stick to what we know and try to figure out what is making the SBC connection drop. That's your best bet for success.