Complex sentence: If no one picks up the parked call after the set time, the telephone system may "ring back" the parked call; i.e., it will transfer the parked call back to the person who originally parked the call.
Split into simpler sentences: If no one picks up the parked call after the set time, the telephone system may "ring back" the parked call. This transfers the parked call back to the person who originally parked the call.