Monday 17 March 2008

VoIP headaches

I've recently signed up with PennyTel to get better prices on phone calls. This was after two relatives of mine both recommended PennyTel and said how easy the whole thing was to set up when using a Linksys SPA-3102.

OK, so I signed up and purchased the Linksys device. I set the networking stuff through the phone then followed the guide on the PennyTel website to configure SIP (VoIP connectivity stuff). I was feeling pretty good about the whole thing, that is until I made the first phone call!

I thought I'd try to impress a mate so I called up one of my tech savvy friends and told them I was using VoIP to talk to them. The quality sounded quite good, then after 32 seconds the call dropped out! I had called a mobile so I thought it may just be a glitch. The next two calls resulted in the same drop out after 32 seconds. By this stage my friend thought it was quite amusing that my new phone service was so unreliable after I had been boasting about the cheap call rates!

After hours of Googling and messages back and forth between PennyTel support, I still hadn't managed to avoid the call drop out, or another intermittent problem where the SIP registration was randomly failing. The settings looked fine, and PennyTel didn't appear to have any outages as I tested things with a soft phone from another DSL connection. I was really regretting the whole thing, and getting pretty pissed off. I had a think about the whole scenario, and the only thing I hadn't eliminated was my DrayTek Vigor 2600We ADSL router. I had already set the port forwards required for the Linksys SPA (UDP 5060-5061 and 16384-16482) so thought nothing more of router configuration. As a last resort, I searched the Internet for people running VoIP through their DrayTek to see if any incompatibilities existed. I came across a site with someone experiencing my exact problem, and they had a workaround! It appears that the 2600We has a SIP application layer proxy enabled by default. This really confuses things on the Linksys and has to be disabled. After telnetting to the device and entering the following command, things were working great:

sys sip_alg 0

Note that you may need to upgrade your DrayTek firmware for this command to be available.

After the changes I made some calls and no longer got disconnected after 32 seconds! Woohoo! At the end of the day I'm glad I chose VoIP for the cost savings, even though it caused me grief the first few days.

Update: One other setting I have found needed a bit of tweaking was the dial plan. Here is my current Brisbane dial plan for an example:


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