Linphone is an open source SIP client with video support. It lets you make SIP/VoIP calls from your Android device. It can be found in the Android market or alternatively it can be downloaded here.
Once Linphone is installed and properly configured, you’ll be able to make or receive calls over WiFi, 3G or EDGE connections. If you choose to make VoIP calls over 3G, check if there are any contractual impediments and make sure that your mobile phone carrier doesn’t block outgoing connections to port 5060.