3CX v18 u& (b312) - Inbound external calls failing

[MarkCSUK]

Hi,

our client has reported that this morning inbound and outbound calls are failing.

the call reaches the ring group, the phones in the group ring once and go straight to voicemail.
outbound calls are not connecting.
internal calls between handsets are working fine.

 

[AWS2P]

So related to your description I guess it's a SIP trunk problem, is it a 3CX supported one?
did you try to contact sip provider support?

Where is located your PBX? local? cloud?
If cloud do you use an SBC on phone location?

 

[MarkCSUK]

Hi,
thanks for the query. The 3CX is on-premise. I did contact the SIP provider they found no issue.
HOWEVER
it appears the Virgin Media router was doing 'something' - a reboot of that and all is working correctly again.

 

[JohnS_3CX]

Hi Mark,

Might want to run the 3CX firewall checker from your management console then.

If there is an underlying issue, it will return when you least expect it.

 

[MarkCSUK]

Hi John,

the client had an internet outage and then when it came back 3CX was playing up (they neglected to mention the internet outage initially) - the router they have has 4G backup this appears to have either not engaged or disengaged properly. The reboot sorted that out. The client is speaking to the ISP about this.

 

[JohnS_3CX]

If it failed over to 4G (or any secondary backup connection really) that would cause issues, the least of which being the public IP changing. 3CX will not be aware that the IP has changed and will use the wrong contact IP in SIP messages going to your trunk provider.

In addition, 4G links tend to use CG-NAT. This means what your port forwards become ineffective because your public IP is behind a NAT you have no control over. All in all, the event you described does justify the issues for the outbound calls not connecting.

Might be a good idea to host their PBX in the cloud. In that case, the internet issues and failover to 4G will not affect you at all as the PBX will be happily continue working in the cloud, and the local phones will connect through it via a 3CX SBC. The moment the IP changes and the connection drops, the SBC will quickly reconnect and you can resume making/receiving new calls.

The bonus is that they can also keep using their lines over their 3CX smartphone app via 4G even if the building's internet and power goes down entirely and the deskphones are rendered inactive.