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We have a 3CX system with over 200 Snom D735 phones. All works well. But recently we purchased 15 new Snom D735's. These 15 D735 phones register fine (auto-provisioned), but after almost exactly 8 min. they unregister with 3CX. If one tries to register then through the phone interface, they try but will never register. If you plug out the POE and power cycle them, they register fine again but after 8 min, they unregister again. That 8 min cycle is very repeatable. All the new phones have exactly the same firmware ( and this firmware has worked well with all our other D735s. This is the maximum FW version approved by 3CX. We have compared the D735 settings between our existing phones and these new phones, and can't see any difference. Also we have tried the phones at a network location that works fine with any of the existing phones. This issue has really perplexed us, so any help from the community or Snom support would be greatly appreciated.


Steve G.

End user

Joined: 26.01.2023


  1. Hello Steve,

    can you still reproduce / observe the reported issue (with the 8 minutes) when the phones are updated to the most recent 3CX supported Firmware: ?

    Thanks and greetings from Berlin,

  2. End user Steve G.

    I have solved the issue. Wireshark showed just at about the 8 minute mark after the D735 tried to register with 3CX again, there were many TCP RST packets with the IP of the D735 as the destination. They didn't come from the 3CX server but from a device on a different part of the network. I tracked down the MAC address and it seemed to be an older Netgear WiFi router, My guess is that it had SIP ALG checked (the default). These SIP packets were corrupting or interfering with the SIP conversation between the D735 and the 3CX. I blocked the Netgear MAC in the firewall, and eventually the user took it off the network since it had no Internet access anymore. Then the problem with all the new D735's then went away. A 3CX partner I knew had warned me about SIP helpers on the network interfering with the SIP conversation, and he was correct. Why it only affected the new D735's and not the old I do not know.