BLF not working anymore D375/D385 w. 3cx 18.0 U5 B418

[phoyer]

After years of operation without problems, the BLF on the Snom phones connected does not work anymore. The phones successfully subscribe to receive updates but 3CX does not provide any status updates. Calls are working fine.

Below an example of a successful subscription:

Sent to Udp:178.63.218.90:5060 from Udp:192.168.20.127:36228 at Nov 17 11:45:04.462 (671 bytes):

SUBSCRIBE sip:[email protected] SIP/2.0
Via: SIP/2.0/UDP 192.168.20.127:36228;branch=z9hG4bK-6lvethti91zq;rport
From: <sip:[email protected]>;tag=8q7zshu3ec
To: <sip:[email protected]>
Call-ID: 313636383638313930343238363637-snhxcwy6es6g
CSeq: 3 SUBSCRIBE
Max-Forwards: 70
User-Agent: snomD375/10.1.64.14
Contact: <sip:[email protected]:36228;line=wvu4jdje>;reg-id=1
Event: dialog
Accept: application/dialog-info+xml
Proxy-Authorization: Digest username="23",realm="3CXPhoneSystem",nonce="414d535c1986a19a33:511e5c639510f29595513a26ba0a8afc",uri="sip:[email protected]",response="245eb6979821e3b6854932abf1fe188f",algorithm=MD5
Expires: 900
Content-Length: 0


Received from Udp:178.63.218.90:5060 on Udp:192.168.20.127:36228 at Nov 17 11:45:04.659 (344 bytes):

SIP/2.0 200 OK
Via: SIP/2.0/UDP 192.168.20.127:36228;branch=z9hG4bK-6lvethti91zq;rport=36228;received=87.9.219.225
Contact: <sip:[email protected]>
To: <sip:[email protected]>;tag=bf29447e
From: <sip:[email protected]>;tag=8q7zshu3ec
Call-ID: 313636383638313930343238363637-snhxcwy6es6g
CSeq: 3 SUBSCRIBE
Expires: 900
Content-Length: 0

In the app, all busy states are shown correctly.

 

[JohnS_3CX]

Is this a STUN phone?

 

[phoyer]

No, STUN is disabled as the router (Zyxel ZyWall) is acting as ALG.

 

[JohnS_3CX]

In this case you are relying entirely on ALG, something we do not support nor recommend because it causes issues exactly like the one you are facing now. It works for a while then stops, making it an unreliable solution.

Consider installing a 3CX SBC if you want these issues to go away: https://www.3cx.com/docs/3cx-tunnel-session-border-controller/

 

[phoyer]

I have now disabled ALG and using STUN. No change. However, the network configuration has remained the same and nothing has changed.

I can do a pcap on the server on which 3cx is running and check if any messages are being sent out. My doubt is no. Before doing that, is there any condition or configuration setting which would prevent 3cx from sending status updates to connected SIP phones?

 

[JohnS_3CX]

Either STUN or ALG is not going to be a permanently stable solution.

Each manufacturer has their own ALG implementation which tries to do the best it can of a bad situation.
STUN on the other hand, requires you to port forward the phone's SIP port and RTP ports so it is exposed to the internet and can receive SIP messages from 3CX. Neither is a good solution and at the end of the day you are at the mercy of the ISP, the Modem, the Phone brand and the firewall (if you use one).

When you get tired of troubleshooting the same issue over and over, give the SBC a try and I think you will be pleasantly surprised.

Alternatively, if this is just one remote phone, consider using the 3CX apps which are also designed to work well behind NAT, ALG and firewalls.

 

[phoyer]

Just as it has always perfectly and without any problem so before spending the work in installing an SBC and then maybe facing the same issue: are you sure that the situation is not caused by a configuration setting or malfunction in the 3cx switch? If not, are there any setting I shall check before or other steps other than a restart which I should try?

 

[JohnS_3CX]

You can compare captures from the PBX and the phone pcap to confirm.

There are no settings on 3CX, its automated and works the same for all phones with Subscribe and Notify messages.