Caller id stoped working I dont know how to clear the old numbers

You're right. The CID buffer is 300. Now, somewhere there's a command to let new calls overwrite the old. Hmmm, why is that?
That's just the buffer size. From the attendant phone on intercom, dial 7373# and the Display will say Incoming Call Log ON. That lets the 301's call overwrite (7372# does not).

THis is the command set for a TA-824. It may be different if you have a TA-624, TAW-848, or any other sytsem that the 7731 may be plugged into.

Carl