I would also try swapping lines 2 and line 5 and see if it stays with line 5. I have seen alarm systems do some strange things.

An alarm system is BEFORE the phone system so if the associated line goes off hook you WON'T have the line buttons on the phones indicate that it is in use.

This is either a bad phone, problem with the wiring, some device like a computer modem on the station side or a bad system module. I'm leaning towards a bad phone or module. Temporarily creating DSS buttons for all extensions like Tommy suggests will tell you where to look.

-Hal

CALIFORNIA PROPOSITION 65 WARNING: Some comments made by me are known to the State of California to cause irreversible brain damage and serious mental disorders leading to confinement.