Telephony problems - connected through SBC - LANCOM

[BrightBlue]

Hello everyone,

Our team is having considerable problems in the current constellation:

3CX v18 (latest version) - Self-hosted at Hetzner (could not post in the correct category)
SBC at each site
LANCOM router (previously converted SIP from the old analogue PBX that has been replaced by 3CX)
Fixed network (YEALINK T57W) and handsets (Gigaset) are affected.

The following symptoms occur:

Calls are dropped during calls
Sometimes you can't hear the other party when you pick up the phone
Sometimes poor call quality

Do you have any ideas as to what could be causing this?
SIP ALG etc. should be irrelevant with an SBC as everything runs via a tunnel, right?
From my perspective, the SBC should eliminate any problems through the tunnel - or are there things I need to consider?
All other customers have no problems, but use Securepoint or Sophos firewalls, also with an SBC.
There is another site of the same customer with Securepoint Firewall, which has no problems.
The SBCs used are Protectli Vault FW6Br2-6, 8GB RAM, 240GB Samsung PM893, so quite decent hardware.

Many thanks for your help!

 

[TheodorosG_3CX]

As per your description this is network related issue. This will require you to check with some captures or check SBC logs.
However the Yealink T57W can also be configured as a router phone, have you tried this option? Trying this might help you eliminate the issue.

 

[BrightBlue]

As per your description this is network related issue. This will require you to check with some captures or check SBC logs.
However the Yealink T57W can also be configured as a router phone, have you tried this option? Trying this might help you eliminate the issue.

Hi Theodoros,
thank you for your quick answer.
Router phone is not an option due to deployment size.
So I gotta check the network.
Is this a common thing with certain firewall manufacturers?
I had actually thought that the SBC deliberately bypasses the firewall and makes such configurations no longer necessary

 

[TheodorosG_3CX]

When using an SBC all traffic will be routed through tunnel using 5090, however if there is any connectivity issue tunnel connection will be dropped. This has to be checked on the network level.