Asterisk is a PBX made into software. It not only runs on Linux, but also BSD, OS X and Windows. Asterisk provides various additional features over the ones already being provided by a PBX. Asterisk does voice over IP using four protocols mainly H.323, SIP, MGCP, SCCP (Cisco Skinny), and also can easily interoperate with almost all standard telephony equipment using inexpensive hardware.

Asterisk also provides various Voicemail services like Directory Assistance, Call Queuing, Call Conferencing, Interactive Voice Responses, Caller ID services, Call Waiting, Call Forwarding, Anonymous call rejection, three-way calling and many others.

We do not need any additional hardware except our telephone for Voice-over-IP. The same VOIP provider can be used both the incoming as well as outgoing calls; they can also be handled by different providers.

Although when we require connecting with analog or digital equipment, then we require many hardware devices that are readily available.

The Inter-Asterisk eXchange(IAX) Voice-over-IP merges the voice and data traffic seamlessly across different networks. It allows advanced integration of information.