What SIP-based IP PBX's are available?

Asterisk - a linux based IP PBX
SIPX - another linux based IP PBX
3CX Phone System - a windows based IP PBX

For a more extensive list of IP PBX software, visit the software directory at WindowsNetworking.

IP PBX, SIP & VOIP common questions 

• VOIP FAQ & Info
• What are SIP Phones?
• What is IVR / interactive voice response?
• What is an auto-attendant?
• What is a voice mail system?
• What are IP Phones / IP Telephones?
• Microsoft Response Point vs 3CX Phone System for Windows
• What is Voice over IP?
• What is a PBX Phone System?
• What is a VoIP Telephone?
• 10 reasons to switch to an IP PBX
• What is Unified Communications?
• What is SIP - Session Initiation Protocol?
• What is SDP - Session Description Protocol?
• What is H323?
• What is ECHO cancellation?
• What is RTP - Real Time Transport Protocol?
• What is RTCP - Real Time Transport Protocol?
• What is a SIP-URI?
• What are SIP Methods / Requests and Responses?
• Can you list all known SIP responses?
• Example of SIP Call session between 2 phones
• How does FAX work in VOIP environments?
• What different types of CODECS are there?
• What is T38?
• What is FOIP - Fax over IP?
• What is DID - Direct Inward Dialing?
• What are the benefits of an IP PBX?
• IP PBX: How an IP PBX / VOIP phone system works
• Sip phones / VOIP Phones types
• VOIP gateway information - Learn about VOIP gateways
• What do the terms FXS and FXO mean?
• What is a STUN Server?
• What is a SIP server?
• What is a good source of VOIP info?
• VOIP-defined
• VOIP Definitions
• What does ENUM mean?
• Check out these VOIP blogs and VOIP links