We have a 8600 quad system which has suddenly started to block genuine callers. Anyone else had this problem? Also both incoming and outgoing sound level has dropped?
Is caller ID still active on the line? If you disable the Call Control & call from your mobile, is the number displayed? There have been a few reports over time of the facility dropping off a line.
If CLI is working as it should then you may have to bite the bullet & reset the base station. Unfortunately that will likely wipe your contacts & blocked lists.