I agree that the battery in the system must have finally died. I have had the same experience on two 616 systems. Best choice is to get a new system board or whole KSU and start over. A good UPS/BBU is also worth considering. I suggest an "on-line" system where the KSU power actually runs on the UPS so that when there is a power failure it does not "transfer" from AC to the UPS. I think it is better especially for the voice mail which I believe is more sensitive to power failures. This also should help if there are surges or quick failures and restores which can prove to be more damaging than a full/complete loss of power. Good luck.

Jim Lynch