Welcome to the board.

First thing to check is to see if the system has been switched to night mode, check your user guide as I am not familiar enough with that system to give you the steps to turn night mode on or off.

Next, an older system like that may lose it's programming and resort to default if it has a power failure, the battery that holds up the program eventually dies and your back at square one. If that's the case the system will need to be reprogrammed to establish the ringing extensions.

Those were just basic troubleshooting ideas, I don't know enough about that system to say more. Maybe someone else will chime in.
Mark

Reliable Telephone Service