Solved Snom M900 DECT ring group issue

[o.assmus]

Hey guys,

i’ve got a strange behavior on ring groups in a 3CX DECT setup.

Environment
Server: Raspberry Pi 4 8GB, Raspian Buster
3CX: 18.0 Update 2 (Build 314) Free Standard License (4SC)
DECT-Base: 1x Snom M900 FW 520.1 (530.2) Single Cell(!)
Handsets: 8x Snom M65 FW 520.1 (530.2)

- no SBC, simple switched network
- 7 extensions (same settings)
- 1 ring group

Codec priority on base
G711a
G722
G729
G711u

Whats working
The provisioning of the base works well. All handsets are registered. I can successfully call all extensions from internal and external - all handsets are ringing.

Error
On all handsets the call from the ring group is shown. When I try to accept the call, e.g. on handset A, the handset quits the call, while the others are still ringing. Then I accept the call on handset B it works without problems. The other way, when accepting a call on handset B from the ring group there are no problems.

The SIP Log shows an error „488 Not Acceptable Here“ for phone A.

13.03.2022 15:39:21 - Leg L:33.9[Extn:636] is terminated: Cause: 488 Not Acceptable Here/INVITE from 10.20.0.10:5060
13.03.2022 15:39:21 - L:33.9[Extn:636] got Terminated Recv 488/INVITE from 10.20.0.10:5060 tid=dfcca12482fbe420 Call-ID=UySqqOzbVFoPIewJ8l2f5w..:
SIP/2.0 488 Not Acceptable Here
Via: SIP/2.0/UDP 10.20.0.5:5060;branch=z9hG4bK-524287-1---dfcca12482fbe420;rport=5060
Max-Forwards: 70
Contact: <sip:[email protected];line=42992>
To: <sip:[email protected]>;tag=s74ao5u9e9o0
From: ":All" <sip:[email protected]:5060;nf=e>;tag=273ac250
Call-ID: UySqqOzbVFoPIewJ8l2f5w..
CSeq: 1 INVITE
User-Agent: snomM900/05.20.0001 (MAC=000413B62D56; SER= 00000; HW=2)
Content-Length: 0

13.03.2022 15:39:21 - Terminated from <sip:[email protected]>;tag=s74ao5u9e9o0 to ":All" <sip:[email protected]:5060;nf=e>;tag=273ac250; reason: Error
13.03.2022 15:39:21 - Call to T:RingAll:800@[Dev:sip:[email protected]:5063;rinstance=034283e2d4421cda,Dev:sip:[email protected]:5060;line=8647,Dev:sip:[email protected]:5060;line=59566,Dev:sip:[email protected]:5060;line=13795,Dev:sip:[email protected]:5060;line=32851,Dev:sip:[email protected]:5060;line=39194,Dev:sip:[email protected]:5060;line=42992] from L:33.1[Line:10001<<012345678901] failed, cause: Cause: 488 Not Acceptable Here/INVITE from 10.20.0.10:5060
13.03.2022 15:39:21 - L:33.9[Extn:636] got Failure: Failure Recv 488/INVITE from 10.20.0.10:5060 tid=dfcca12482fbe420 Call-ID=UySqqOzbVFoPIewJ8l2f5w..:
SIP/2.0 488 Not Acceptable Here
Via: SIP/2.0/UDP 10.20.0.5:5060;branch=z9hG4bK-524287-1---dfcca12482fbe420;rport=5060
Max-Forwards: 70
Contact: <sip:[email protected];line=42992>
To: <sip:[email protected]>;tag=s74ao5u9e9o0
From: ":All" <sip:[email protected]:5060;nf=e>;tag=273ac250
Call-ID: UySqqOzbVFoPIewJ8l2f5w..
CSeq: 1 INVITE
User-Agent: snomM900/05.20.0001 (MAC=000413B62D56; SER= 00000; HW=2)
Content-Length: 0

13.03.2022 15:39:21 - [CM503003]: Call(C:33): Call to <sip:[email protected]:0> has failed; Cause: 488 Not Acceptable Here/INVITE from 10.20.0.10:5060
13.03.2022 15:39:21 - Session 2932 has failed in leg L:33.9[Extn:636] ; Cause: 488 Not Acceptable Here/INVITE from 10.20.0.10:5060
13.03.2022 15:39:21 - Failure from <sip:[email protected]>;tag=s74ao5u9e9o0 to ":All" <sip:[email protected]:5060;nf=e>;tag=273ac250

What I’ve tried so far
- switched firmware on handsets an base between 520.1 an 530.2
- switched PBX audio delivery on/off
- captured traffic from 3CX and the case (still analyzing)
- reregistered the handsets


Maybe others with an equal setup can give some hints. You'll find the full SIP Log example from an external call attached. There the extension 636 has the error

Thanks,
Oliver

 

[JohnS_3CX]

Hi,

It looks like the M900 is rejecting the call. You appear to be sending the call 631 (which is part of a ring group) and then forwarding the call to 800. I think you should avoid this for now, and send the call directly from the trunk to 800 because this is just complicating the problem.


Why is the device rejecting the call?

The logs do not tell us why the M900 is rejecting the calls. We can test a small theory I have:

These DECT devices can have multiple extensions registered BUT can only have a few active calls at a time.

If you are "ringing" more extensions than the base can handle, I think it may start rejecting calls. Once a call gets rejected, you now have less extensions ringing and you are probably within the acceptable limits of the base.


What to test

1. I think if you delete 2 extensions so that the total is now 5, and try your test again, the problem might go away.

2. If the above fixes it, but you still need to have 7 extensions, you can alternatively switch to a lower quality codec by editing the M900 settings in FXS/DECT page, and trying G729 as the top codec. Reboot it to reprovision itself and make sure you are testing 5.30 (not 5.20 - this is old now)

 

[o.assmus]

Hello JohnS_3CX,

thank you for your considerations.

I found on the snom wiki that the M900 can only handle 5 simultaneous calls with HD codecs. Snom Wiki

Codec priority seems to be ignored so all calls are made using G722. On the M900 syslog I can see the capacity issue.

My confusion was that all the handsets were ringing. I thought that the unused lines are released after accepting a call.

After only selecting G711a and G711u as codecs, everything works fine.

Thank you!

 

[JohnS_3CX]

Glad to hear you were able to confirm it and solved with a codec change.