Agree with the above.
Listen to what kind of ring you are getting, look in the manual (and the device's configuration) and determine what kind of call is causing it, -- X_DefaultRing, X_CallOnHoldRing, X_RepeatDialRing, X_BargeInRing, X_CallParkedRing.
Also, look in the device call history to see if you can deduce what kind of call is causing it.