When you're out in the field and need to call in a favor from a fellow agent or report back to HQ, you don't want to depend on the public phone network if you can avoid it. Landlines and cell phones alike can be tapped by all sorts of shady characters and to add insult to injury, you have to pay good money for this service. We can do better.

Welcome to the wonderful world of Voice over IP (VoIP). VoIP is a blanket term for any technology capable of delivering speech between two end users over IP networks. There are plenty of services and protocols out there that try to meet this demand, most of which force you to connect through a central server that you don't own or control.

We're going to turn the Pi into the central server of our very own phone network. To aid us with this task, we'll deploy GNU SIP Witch—a peer-to-peer VoIP server that uses Session Initiation Protocol (SIP) to route calls between phones.

While there are many excellent VoIP servers available (Asterisk...