VOIP shenanigans :(

[synaesthesia]

Hoping someone is an absolute expert in this, because I'm going greyer quicker than Gandalf.

 

We have a leased line through a 3rd party. Line provided by Gamma.

We have a Splicecom VOIP system provided by a supplier, the SIP trunk is also provided by Gamma. Unrelated contracts.

 

We've just gone live on said leased line with a Smoothwall. Everything works fine apart from the phones - no audio. Calls connect but no audio in either direction. Internally it's fine.

This is screaming a NAT issue to me, so we've had a lot of back and forth trying to get this sorted, all are certain it's not their problem. Gamma have provided me with packet captures showing their SIP provider is getting our internal IP rather than external. Again, NAT issue.

But all appears correct on Smoothwall. Enabled and disabled the SIP helper which I know do the opposite of help. The only other thing inbetween is the router provided by Gamma on the leased line, and they're certain it's not that.

It all worked fine on the original leased line. I'm absolutely at a loss.

Any pearls of wisdom?

[Davit2005]

Disable SIP ALG on the firewall?? Ah I see you have already done that, lol.

[synaesthesia]

Disable SIP ALG on the firewall?? Ah I see you have already done that, lol.

 

Funnily enough I'm not convinced this is the case on the Cisco router to which we have no access. Many, many many reports of that being an issue on Cisco stuff, but Gamma say it's fine. Whether that means it's disabled or it should work is beyond me.

[Wave9_Tim]

I've seen things like this a few times before and there's a few suspects it could be.

 

You've already sorted out the SIP ALG one (I'm assuming Gamma aren't lying about it being off their router, but sometimes they are mistaken). There's also the external IP setting on the PBX which I seen often to be the cause (in the SIP trunk definition normally).

 

If that is set to anything other that the external NAT RTP goes missing in action. You can check this by running a TCPDUMP on the SIP packets during the call setup (I'd usually do this from the firewall, not sure if you can on a Smoothwall). I don't have an example to hand but during the INVITE stage you should see the external NAT IP showing up, if you post/send me that I can check if you'd like. If it's the internal, private IP of the PBX then that's the issue. You can also see this behaviour when the external NAT and firewall rules aren't allowing the right UDP packet range or is NAT'ing the traffic to the wrong internal IP. It's usually one (or more) of those. You can also TCPDUMP the RTP stream packets (which I presume are being generated but is worth checking) and see what's going on. Gamma will use a seperate IP for the RTP host so it's easy to do (and you should see a stream of UDP packets). That's a rough overview, hopefully one will either sort the problem out or point you in a better direction.

 

Regards

[Davit2005]

I done something stupid. Wondered why my home 3cx had no audio outbound. Teams was hogging my audio device preventing 3cx from accessing it for the microphone. But yep I have seen SIP ALG issue not only on Cisco. Edited October 25, 2023 by Davit2005

[synaesthesia]

Whelp, so it's resolved! The comms provider hadn't thought about a setting. I asked if they could remote on and show me the settings as we don't have access, which they duly did. They only usually change the external IP when internet line is changed, but this needed a little more. I noticed a "Nat mode" option, pointed this out and set to Static and bingo!

Lots of work and hassle over a setting they'd never seen before

Next step for this.... look at 3CX