Currently baresip app supports SIP voice calls and messages with TLS
transport, voicemail Message Waiting Indication (MWI), incoming call
transfers via REFER method, PCMU/PCMA and opus voice codecs, as well as
ZRTP and (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.