Avaya 3641 and 3645 (Spectralink/Polycom 8020 and 8030) wireless handsets
Handsets appear to randomly power off, even when fully charged.
Likely caused by loose or damaged connectors from the battery pack to the connectors inside the phone itself. Can be caused through general age of the handset, improper cleaning and of course, dropping the handset.
Replacing the battery would be the first port of call, but if it is the phone connectors itself that are damaged, a replacement handset would need to be sourced.