I had been using the X-Lite phone for some time and it worked great, then one day it would no longer register and connect to my SIP account. I checked a lot of things but could never definitively identify the problem or event that caused it to stop working. The Counterpath forum is no help and the moderator there is in denial that there could possibly be a problem with "their" software... although there are many people having the same connection problem. My best guess is that the constant "security upgrades" that MS is adding to the XP OS are at the heart of the problem... something changed that X-Lite did not like. Anyhow, a friend suggested that I change phones and try the 3CX. I downloaded it, installed it, configured the account and it came right up and connected without a problem. The selection of audio codecs is a bit sparse but the main thing is that it does work and has been hassel-free so far. I did nothing special with the Windows firewall or the router ports... it just worked. I see a lot of people chasing ther tails on the web trying to solve connection issues with their softphones and getting nowhere. The free phones are nice but there is little or no support when they do not work. Try the 3CX, it worked for me.