What you are planning to do is on the computing area with its own stability. It is as stable as you make it. Most of the time its not the vendors fault if there is instability in the IP PBX area.
Another stable system is the HiPath 5300-Server from Siemens – and old dragon in the PBX market. They might know what stability means. It is based on a Windows NT embedded in the Flash EPROM.
More information under:
http://www.icn.siemens.com/hipath/productsServices/hipath5000.html