How old is your phone system?
Is it the Panasonic that rings sometimes for no apparent reason?
Also when it rings does anything show up on the display?
May I have your town and state so I can locate a good technician for you.
Or if you wish I can call you and its possible I may be able to solve this for you over the phone as I have a good suspicion of the problem.
I will send you a phone service offer if you accept I will call you at your convenience. If not, then we can continue like this.;-)