The worse affected extensions are indeed 10-18 on the processor module. Unplugging 2-3 phones from just the processor module solves the problem. So I would agree with the power supply being the likely culprit. There is also one extension on the 308EC that experiences the same issue from time to time.

Maybe this is also why the overall volume, particularly the outgoing volume on the CO lines is low. It did not used to be that way. We have had some callers complain they have a hard time hearing us.

Might just be time to swap in a IP Office system....