SIP phone has no audio at either end

[Greg Dent]

Hi all,

We have recently started to deploy SIP phones to some of our project sites and offices. In one particular case, which is now doing my head in, I cannot seem to get the audio to work at all. The phone rings and makes an initial connection, but after that, fail - no audio on either end.

The SIP trunk is actually provided over the internet by a 3rd party provider (Gamma), so all the phones need is a local drop off to the internet. I have configured my VLAN on the core router to do this. Each port on the trunked Cisco switch has the voice vlan configured to look at the new voice VLAN I created on the router.

Long story short, the VLAN and connectivity is fine and works - the SIP phones were able to connect to Gamma's servers and pull down their updates and configuration.

Next step was to reconfigure the ACL and stateful packet inspection to allow SIP and RTSP to the desired destination servers (in this case, Gamma's SIP servers and networks). This was completed on a handful of other routers, and works ok on those. There was also some specific ports I had to open on UDP which has also been done.

Then we have to turn off Cisco SIP ALG, by using the command "no ip nat service sip tcp port 5060" - otherwise the router will try and do some funky stuff with the voice packets.

All done. Great, I thought.

But we get this problem where the SIP phone rings when dialled from an external source (which in this case is everything, since these are 3rd-party hosted SIP phones), but neither end can hear one another.

It tells me that once the intial connection is established over the data network, the voice packets are getting destroyed by something. In order to troubleshoot this, I turned off the stateful packet inspection AND access-lists. Still getting the same issue.

The router in question is a Cisco 1801 running IOS 12.4(24(T8). Connectivity is provided by a BT ADSL connected directly into this router.

Any help much appreciated.

[paolo bevilacqua]

If you disable SIP ALG, you can be 100% sure that voice will not go through.

You can update IOS, and re-enable it.

[Greg Dent]

Why is that?

Our SIP provider specifically states that we need to turn this off.

Could you provide an explanation for this? I've read that it interferes with most VOIP standards, also we have this working on 3 of our project sites already with SIP ALG turned off.

I R Confused.

[paolo bevilacqua]

No SIP ALG = no adjustment of IP address in payload, no dynamicport  opening, no NAT translation creation for RTP flows = nothing works.

Is not like Cisco left it as default just for fun. 

[Greg Dent]

So why do the other site SIP phones work?

And why are our providers saying the complete opposite to you? We even have to disable SIP ALG on our BT 2Wire routers where we have SIP phones.

I need to go on a VOIP course I think.

[paolo bevilacqua]

I don't know about other brand routers but I I know about my customers sites that use Cisco routers.

They all work perfectly with recent IOS, and things left at default. That includes SIP phones, SIP PBX and whatever else.

[Greg Dent]

We're a Cisco house, so will try turning it on. Let you know how we do.

Our BT routers are on some smaller sites that only have ADSL and no MPLS link etc etc.

[Greg Dent]

We resolved the issues.

They were twofold.

Firstly, the user  on site had not connected the SIP phone to the correct port (on the  phone!) - it was in the AUX port, which meant it got data and rang, but  voice woudlnt work.

Secondly, our SIP provider decided to switchover to  their new servers this morning and didnt feel like telling us! This  meant that our existing ACL's were incorrect, so we updated those and  all working!

I would add that SIP ALG is still DISABLED  on all our Cisco routers, so I dont know where you're getting your info  from. Maybe it is just our provider in particular that uses  non-standard SIP ALG protocols.

Appreciate the efforts anyway.

Cheers.

[paolo bevilacqua]

I would add that SIP ALG is still DISABLED  on all our Cisco routers, so I dont know where you're getting your info  from. Maybe it is just our provider in particular that uses  non-standard SIP ALG protocols.

I don't "get info", I have first hand experience.

Just for curiosity, you can try restoring the thing to default, and let us know if it works or not.

[Greg Dent]

Thats fair enough

I will try it as soon as I have a development environment to do it in. These are all live sites I'm playing with, and since they are now working I can't really risk breaking them again.

thanks

[paolo bevilacqua]

Greg Dent wrote:
Thats fair enough 
I will try it as soon as I have a development environment to do it in. These are all live sites I'm playing with, and since they are now working I can't really risk breaking them again.

As they work now with SIP ALG disabled, in case of problems when re-enabling it for a test, you would then just revert config.

[Leo Laohoo]

Then we have to turn off Cisco SIP ALG, by using the command "no ip nat service sip tcp port 5060" - otherwise the router will try and do some funky stuff with the voice packets.

My VoIP provider insists I DISABLE ALG on my 880.  And it works, however, your command is missing.  Have you entered "no ip nat service sip udp port 5060"?

[paolo bevilacqua]

Leo Laohoo wrote:
Then we have to turn off Cisco SIP ALG, by using the command "no ip nat service sip tcp port 5060" - otherwise the router will try and do some funky stuff with the voice packets.
My VoIP provider insists I DISABLE ALG on my 880.  And it works, however, your command is missing.  Have you entered "no ip nat service sip udp port 5060"?

Good observation, but it could be that the OP ITSP uses TCP, in that case the command he mentioned would be correct.

[Leo Laohoo]

Good observation, but it could be that the OP ITSP uses TCP, in that case the command he mentioned would be correct.

Sorry, Paolo.  Didn't mean to say you were wrong. 

True.  But I thought that if one was to disable ALG you need BOTH commands.