Belle-sip is a SIP (RFC3261) implementation written in C, with an object
oriented API.

* RFC3261 compliant implementation of SIP parser, writer, transactions and
  dialog layers
* HTTP client API
* support of client TLS certificate
* fully asynchronous transport layer (UDP, TCP, TLS)
* fully asynchronous DNS resolution with SRV
* full dual-stack IPv6 support
* SIP transaction state machines with latest corrections (RFC6026)
* automatic management of request refreshes with network disconnection
  resiliency thanks to the "refresher" object
* supported platforms: Linux, Mac OSX, Windows XP+, iOS, Android,
  Blackberry 10

WWW: https://www.linphone.org/technical-corner/belle-sip.html