Running two Engin voiceboxes on the same network

Again, a post that'll only help a very few of you, but for the sake of those few...

Engin is a fabulous VoIP (internet telephony) company. We have two accounts with them - one for our business, and one for our home.

Everything was working perfectly at our old address, with TPG as our ISP.

We've relocated, and TPG doesn't cover our outlying suburb, so we're now with Telstra.

Problem : With Telstra's ADSL2+ modem, only one of our VoIP lines works properly.

Specifically, both make outbound calls just fine, but inbound calls only work consistently for one of the two lines.

If I reset the Engin voiceboxes (which internally are SipuraSPAs), and wait a few seconds for them to re-register with Engin, then inbound calls work fine for both lines, for about five minutes, and after five minutes, its just the one line that inbound calls work for.

I contacted Engin tech support last week, and they changed one of my Engin voiceboxes to use the "bring-your-own" Engin settings. I thought that fixed the problem, but it turned out that the test call had been made within five minutes of a "soft reset" of the Engin device, and that's the only reason the test inbound call worked at that time. i.e. problem not solved.

Today I got it working, at last, and permanently.

It's not intuitive, but I set up two port forwarding ranges on my ADSL2+ modem, one for UDP ports 16384 through 18383, and the other for UDP ports 18384 through 20384. I pointed one port forwarding range to the one Engin voicebox, and the other range to the other voicebox.

It's about half an hour later, and inbound calls for both lines are still working perfectly. Nice...

In my previous attempts to resolve this issue, Google-ing for "two Engin voiceboxes" and such like turned up a grand total of zero people discussing problems with running two Engin voiceboxes side-by-side. So hopefully, for the next person who needs a little help, this post might provide a few ideas...

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UPDATE : A few hours later, and I discovered it's NOT working, but I figured out why, and here's the low-down :

It turns out that the port forwarding DIDN'T HELP ONE BIT.

But I've pinpointed exactly what the problem is :

The Engin voiceboxes re-register with Engin every nearly-30-minutes. (It's every roughly 29 and a half minutes.)

But the Telstra ADSL2+ router closes UDP "connections" after about 300 seconds (five minutes) of inactivity.

So every half hour, there is a five minute window of opportunity during which inbound calls will work, and then nearly 25 minutes where they won't.

My confirmation test earlier today must have been in that five minute window of opportunity.

The fix?

Well, I trawled through the options in the admin panel for this Telstra ADSL2+ modem, and don't seem to have any power to change the default timeout.

So I contacted Engin tech support again, and asked them to remotely log in to my machine and change the default timeout on both my Engin voiceboxes down to 280 seconds. That gives them roughly 20 seconds (far more than enough) to execute their re-registration process before the ADSL2+ router deems their UDP "connection" disposable. The re-registration process resets the modem's five minute timer, and this process, continually repeating itself, twelve times an hour, ensures that inbound Engin calls will work at any time.

And I've removed those ADSL2+ port forwards, as they are now completely useless...