Finally a SIP/VoIP program for the Android OS!
Sipdroid is a great GPL licensed program that allows you to make and receive SIP and make VoIP from your mobile. The app is incredibly simple to use, which just requires you to enter your SIP credentials (server, login and password).
Once it is on, you will see a little green bubble on the status bar indicating that you’re online. After that, just enter the phone number you’d like or use the phone numbers on your contact list, remembering to enter the country code.
The only detriment for now is that it connects only through wifi, so no 3G data connections while you’re out-and-about.