SipDroid is an open source SIP client that allows you to make calls via SIP/VOIP on an Android device. SipDroid can be found in the Android Market or alternatively you can download it here.

Once installed and properly configured, you’ll be able to make or receive calls over WiFi, 3G or EDGE connections. SipDroid allows you to choose the network from WiFi, 3G or EDGE. If you want to make VoIP calls over 3G, check whether there is no contractual limitation and make sure that your mobile phone carrier does not block connection to port 5060.