What do I need? - All you need to get started with VoIP is a broadband internet connection, a PC with microphone & speakers (or ideally, a headset with mic) and a softphone. Once you have got to grips with that you might want to move on to a hardphone or an Analgue Telephone Adapter (ATA) that will allow you to connect an ordinary phone.

The open source Asterisk project offers full PBX capabilities for your VoIP system, allowing calls to be placed between extensions, via VoIP service providers or via your PSTN line. Asterisk allows you to implement auto attendant features (press 1 for foo, press 2 for bar etc), voicemail, voicemail to email and much much more. See my Home Automation page for an example of some of the more unusual things Asterisk can do.

There are several VoIP Protocols but the two which you should consider are SIP and IAX:

SIP: Session Initiation Protocol - The IETF protocol for VOIP and other text and multimedia sessions, for SIP VOIP calls the actual voice packets are sent using RTP. SIP is the most common VoIP protocol today but is not very NAT friendly.

IAX: Inter Asterisk eXchange Protocol - A NAT friendly protocol (uses a single UDP port) originating from the Asterisk project. Mainly used for linking asterisk PBXs although there are now some IAX softphones available and the Digium IAXy ATA is starting to become available now.

• VoIP Providers
• VoIP Softphones
• VoIP Hardphones and ATAs
  

• The VOIP Wiki - a reference guide to all things VOIP
• Voip User
• Unofficial Asterisk Forums
  
• FWD Forums
  
• VoIP.org.uk
  
• BroadbandReports.com
  
• Voxilla
• E164.org - ENUM (DNS for phone numbers)