From Helpful

This article/section is a stub — probably a pile of half-sorted notes and assertions some of which may well be wrong, and not verified as a whole. Feel free to add or refine.

Asterisk is a software PBX. This means it can do calls in and out of

• Local (network-based) VoIP-style networks
• Other VoIP-style networks (Asterisk or not; currently by H.323, SIP, IAX, or ADSI)
• (given the proper hardware) Call in and out to local hardware phone networks
• (given the proper hardware) Call in and out to the public phone network.
• Just itself, if you want to provide this just for friends. Can be useful over server-mediated VoIP when you're on a LAN.

It does some fancy alias/phone number remapping, so that you can do equivalents of things like calling out.

See also:

• Introduction
• Application command reference