Have you verified that the system is still running on AC power and not batteries? The two phones are probably attendant positions. This is usually where this type of alarm shows up. You may not be able to clear it if the system is running on battery. I'll do some research and post anything I find.
What you have is a regular scheduled maintenance alarm. Someone set this up to alert when the batteries should be replaced.
See MB's 90-53-01 to clear it. It can only be cleared through telephone programming methods.
90-51 to set it up.
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