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Albany Creek, Queensland, Australia
07 3882 2933

Why are my calls dropping out?


Earlier in the week a long-term education customer contact us to say that internal and external calls were dropping off so, of course, the question was “why are my calls dropping out?”

We had recently completed a major upgrade for them. They have an Avaya IP500v2 that was running Release 10.1 and we took it up to the current R11.1. We also moved the application server off an old Windows Server they wanted to decommission and put it onto a new Nutanix server.

They also have a redundant chassis which needed to be upgraded with the main chassis. The idea of this is if the main chassis fails then all the phones fail over to this backup chassis and you simply turn the SIP trunk on and hopefully any outage is quite short.

In order to track down any fault like this is has to be narrowed down. On first glance if both internal and external calls are dropping then it might be the phone system chassis itself or perhaps one of the circuit cards.

Remotely monitoring the system was inconclusive so I called the customer to discuss. I asked if they had noticed calls dropping off after a particular time, whether it was limited to inbound or outbound calls or if any changes had been made to the firewall as they have a fairly restricted firewall. They hadn’t noticed it occurring after any particular time, it seemed to occur on both inbound and outbound calls and apparently nothing had been done to the firewall recently. I checked that keep-alives were turned on to keep a hole in the firewall for the duration of the call.

As we were talking the call went silent. It did not drop out though so that was a clue because it had occurred at just after 3 minutes. Unfortunately I had the Monitor paused so I couldn’t see what happened to the call. I called back in with the Monitor running. At just after 3 minutes we ended up with 1-way speech – I could hear them but they couldn’t hear me. I could see that the Internet port had changed. Could it be our old enemy SIP ALG in the firewall messing things up?

I called the customer back and suggested that SIP ALG might be turned on in the firewall and then I got the whole story: they had an issue with the firewall a week or so ago where a firmware update broke it so they rolled it back to the previous version. I don’t know whether that had turned SIP ALG on or whether the config they used had it turned on but after turning that off we had a successful call of over 12 minutes.

The SIP Application Level Gateway is one of a number of ALG’s present in firewalls that are meant to help things. It inspects any VoIP traffic to prevent problems caused by firewalls and if necessary modifies the VoIP packets. In our experience only 1 in 1000 times does it actually help and should automatically be turned off.  

ICS Technologies is based in Brisbane, Australia. Contact us for assistance with your Avaya IP Office.

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