Since I found Michel de Boer’s explanation of many of the fundamental SIP concepts in his 2005 dutch presentation of the Twinkle SIP phone very short and easy to follow - contrary to a lot of other material about SIP out there - I’ve translated it to english.

Here it is: Twinkle - a SIP softphone for Linux

Since upgrading to Karmic I had the problem under Ubuntu that SIP calls under Twinkle would start get all chopped up every few minutes.

Turns out it’s NetworkManager that scans the wireless spectrum every two minutes for networks even if you are allready connected to some AP.

While people seem to be fixing the problem in the proper way, by making scanning interfere less with an active connection, there’s a good soul that provides patched versions of NetworkManager that gives up scanning while there’s an active connection to an AP.

See https://bugs.launchpad.net/ubuntu/+source/network-manager/+bug/373680