Its free sip-client from Linphone open source code .
Offers free audio/video calls and text messaging over wifi or 3g/4g internet connection.Work with varios sip-servers ( asterisk, sipxcom and etc..)
key features:
* high definition audio and video calls .
* audio conference calls with various participants
* pictures and file sharing
* secure communication (TLS, SRTP, ZRTP support)