VOIP Issues

[nathan]

I have a Elastix phone system in one of our primary schools with 2 SIP channels provided by Spitfire with 1 DDI.

 

I have 2 handsets in the School Office. Phone 1 works fine on an external call, but if phone 1 is on a external call and phone 2 attempts to answer an external call - all lines cut off.

 

Also we have a auto attendant setup with an option to be transferred to the kitchen (forward to a simple extension number), this rings twice and then goes silent.

 

Everything else seems to work fine.

 

I've checked the logs and do have entries saying the following;

 

[2017-02-07 11:18:52] WARNING[2300] chan_sip.c: Hanging up call 407711378-23003@SVIGateway - no reply to our critical packet (see https://wiki.asterisk.org/wiki/display/AST/SIP+Retransmissions).

 

This is normally a NAT issue, but it's strange because it seems to work fine on Office phone 1.

 

Does anyone have any ideas?

[SchoolsBroadband]

Sounds like a phone system problem but just in case ask our guys to do a debug on the traffic via the firewall if this goes out via our Internet connection to see if we can see anything odd.

 

Thanks

 

Dave

[Edu-IT]

Does it happen the other way round too? If phone 2 is on external call and phone 1 answers?

[Geoff]

This is a firewall issue. Most likely you have sort of SIP 'feature' turned on in the firewall that can't handle rewrting two SIP channels at the same time. It's usually called 'SIP ALG' or something.

 

https://www.voip-info.org/wiki/view/Routers+SIP+ALG

Edited February 8, 2017 by Geoff

[nathan]

This is a firewall issue. Most likely you have sort of SIP 'feature' turned on in the firewall that can't handle rewrting two SIP channels at the same time. It's usually called 'SIP ALG' or something.

We have a managed firewall that had ALG enabled. I've since asked them to disable this and we are still having the same issue.

 

Surely it would happen with all calls if it was a firewall issue though? Calls are fine to the Office phone 1

[Geoff]

Depends how buggy the firewall is. Turn the debugging on and post the SIP conversation. You may also need to get wireshark out and actually inspect the packet headers at various points on your network.

[nathan]

It's a host fortigate. I'm at the school in the morning so will investigate more.