There are a lot of systems out there (some popular and some not). Your curent setup/system could be made more reliable with an Uninterruptible power supply. A UPS would prevent volitale memory drops and protect your programming.
I'm a nortel tech - so I'm biased, but check out this link for systems that match your requirements.
http://nortel.pbxinfo.com/index.php?...isplay&ceid=27 You'll need to do a little research to determine which system matches the functionality that you desire. Most systems are proprietary, so you'll need to purchase new digital sets, but you can most likely reuse your analog sets.
Chas2002