NetBSD SIP VoIP Software

Asterix - a software of a telephone PBX. Enables you to attach a telephone and make calls, and connect to other telephone services. 
- Features:  Voice mail, conference calling, interactive voice response, and automatic call distribution. You may create new functionality, and add custom loadable modules in C.
./comms/asterisk

OpenSIPS - another SIP Proxy. Made to handle large volume of calls, load balance communications, solve NAT scenarios. Not capable of any media related services. Cannot create systems like VoiceMail or Voice recognition. Uniquely has an ability to load balance VoIP calls. Its a software you use when you have a special need, like when managing large volumes.
./wip/opensips

Linphone - open-source voip software, makes use of the SIP protocol. NAT Friendly, has an address book, multiple SIP proxy support, test IM with notifications, digest authentication, supports IPv6 and multiple calls simultaneously with features like, hold, resume, transfer.You can use plugins to add new codecs, has audio conferencing. 
./wip/linphone

Mumble - open source, low-latency, high quality voice chat used for gaming. Has Opus codec, hold-time for Push-To-Talk, audio cues, Text to speech, "intelligent" chat log scrolling. Great quality, no invite, just connect and discuss.

Teamspeak - A quality scalable app. Consists of client and server software.  Server acts as a host to multiple client connections, capable of handling thousand of users. Great for teleconferencing.
./wip/teamspeak