V-Call, you can use it to contact anyone over the internet with voice.
Features
*Support background mode: you dont need to worry about missing call problem, the program will run in the background
* SIP proxy support: when you cant register directly via network, you can set a outbound proxy to achieve.
* Call history, Contact, Location and so on.
* Best voice quality with codecs: speex, G711, ILBC, GSM
* Continued use of GPS running in the background can dramatically decrease battery life.