We think sipX is the leading open source IP PBX in terms of scalability, robustness and ease of use. The sipX IP PBX has been successfully deployed in a lot of places. The largest known installation serves more than 5,000 users connected to one redundant (HA) system. Small installations go all the way down to a few users served by very low cost hardware.

• You need an IP PBX that is robust, stable and easy to use
• Need a system that offers plug & play management of all the components including phones
• It is a mission critical system and therefore you insist in high-availability redundant configuration. A server failure should not interrupt calls
•  Voice quality is important to you and therefore you want a system that routes media peer-to-peer and not through the PBX
• SIP standards compliance that allows flexible call routing including SIP trunking
• Need scalability - sipX is known to work for deployments in excess of 5,000 users connected to one redundant server
• Lots of features - of course, who does not want that
• Unlimited number of simultaneous calls, and unlimited number of trunks using distributed gateways
• Need IT integration with no special needs for specific HW. Runs on off-the-shelf Intel servers