Baresip app implements a very basic, secure, and fully open source VoIP User Agent based on baresip
https://github.com/alfredh/baresip SIP library .
Currently baresip app supports voice calls and text messages, voicemail Message Waiting Indication (MWI), and incoming call transfers via REFER method. Voice is coded with PCMU/PCMA , G.722.1, iLBC, or Opus codecs. Security is achieved via TLS signaling transport and ZRTP or (DTLS) SRTP media encapsulation.
Development of baresip app is motivated by need for a secure, open source SIP based VoIP User Agent for Android that does not depend on proprietary, third party push notification services.
Source code is available at GitHub
https://github.com/juha-h/baresip-studio, where also issues can be reported.