Baresip Android app implements a very basic, secure, and fully open source SIP User Agent (client) based on baresip library (https://github.com/alfredh/baresip) .
Currently baresip app supports VoIP calls and text messages, voicemail Message Waiting Indication (MWI), and incoming call transfers via REFER method. Voice is coded with PCMU/PCMA 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 User Agent for Android that does not depend on proprietary, third party push notification services.
Source code is available at https://github.com/juha-h/baresip-studio.