I continue to be impressed with the features found in the Windows-based phone system pbxnsip! Just today I got a notification that all the Lines on a gateway were in use--VERY proactive!
This along with 15 other common phone system scenarios can alert the pbx admin by email of a situation that needs attention. pbxnsip also allows you to monitor it using industry standard SMNP alerting tools for many indicators.
We continue to be impressed with the pbxnsip phone system after over a year selling it and a lot of systems in the field. Pleasant surprises pop up almost every week!