Asterisk is a complete IP PBX in software. It provides all of the
features you would expect from a PBX including many advanced features
that are often associated with high end (and high cost) proprietary
PBXs, such as Voicemail services with Directory, Call Conferencing,
Interactive Voice Response, and Call Queuing.

Asterisk supports Voice over IP in many protocols, and can
interoperate with almost all standards-based telephony equipment
using relatively inexpensive hardware.

Flavours:

	IMAP - use imap for voicemail storage instead of files.