What is happening here is that there is a wrong time set by the phone company (usually related to daylight savings time) and every time the phone rings or the system checks for messages, it is getting an incorrect time signal on the caller id, and updating the phone incorrectly.
To change this, you just need to disable the auto time set, and then re-set the time manually.
Grab a handset and key in Menu#2260 Save Off
Then set the time again, and the setting should stick.
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