That software does work, in the sense that it will make and receive calls, however it seems to disconnect SIP when the screen times out or is locked. I've been doing some research and have found some open source software called boghe which has tried to make use of Microsoft's implementation of SIP https://code.google.com/p/boghe/ -- however, you will need to install visual studio for windows phone to build it and depending on your phone (if it has no sd card) get a windows mobile developer account ($99/usd per year or free for dreamspark users) to deploy the application as nobody has released it to the windows store yet. I am continuing to try and play with it as it seems promising as it integrates with the phone and treats SIP calls as ordinary calls, however it does not seem to be able to receive calls- only make them - with 2talk. I am assuming this is a software bug as the above application received calls fine.
It probably 'disconnects' because when the screen goes off it puts the radio's to sleep and the SIP registration expires. You need to make sure data is not turned off when the phone goes on standby but it will reduce your battery life a bit.
There is a bit of stuff around about using SIP over TCP but I don't know if it helps as I have never tried it.