Before we start digging in, let's go over a little base vocabulary of some terms you will see throughout the primer or in other tutorials.

DID - Direct Inward Dialing
A term usually misused as a noun to refer to a public telephone number.

POTS - Plain Old Telephone Service
The typical, basic analog telephone line that runs into your house.

PSTN - Public Switched Telephone Network
The telephone network system of the world at large.  If a PBX was a LAN, the PSTN is the WAN.

ITSP - Internet Telephone Service Provider
A VoIP provider that serves as the gateway between you and the PSTN.

RTP - Realtime Transport Protocol
The protocol used to transmit/receive the actual audio data of a call.

SIP - Session Initiation Protocol
The most common VoIP control protocol, which handles call setup, tear down, and otherwise coordinates the communication between endpoints.