FreeSWITCH is an open source telephony application written in C, built from the ground up and designed to take advantage of as many existing software libraries as possible.  FreeSWITCH makes it possible to build an open source PBX system or an open source voip switching platform as well as unite various technologies such as SIP H.323, IAX2, LDAP, Zeroconf, XMPP / Jingle etc.  FreeSWITCH can also be used to interface with other open source PBX systems such as OpenPBX, Bayonne, YATE or Asterisk

One of our fundamental goals is to leverage as much expertise from the open source community and, if at all possible, to prevent duplication of work.  That said, it is sad but true that not all existing software libraries fit the bill but we try as hard as possible to make it happen.