Walkietalkie is a lightweight application for voice-over-IP
communications between PCs and Macs. It is low bandwidth
low latency, and ideal for voice chat during netgames. It
require any servers or other infrastructure.
pjsip is a multimedia communication library based on the SIP protocol.
It is integrated with a rich media and a NAT traversal library
supporting the ICE protocol. It is very portable and has a small
footprint for embedded use.