Abstract: Particular keys are provided with locking arms that may or may not enter a duct filled with balls and being displaceable in the duct only if a locking element is not inserted. That element will be inserted when keys having said locking arms must not be depressed. The locking element is either electromagnetically or mechanically actuated, particularly for retracting it when the lock is to be released, e.g., following actuation of a legal key. The arrangement is explained with reference to a telephone in which handset lifting and replacing perform initialization and/or restauring functions. The unlocking may be initiated by pressing a legal key having actuation arms which will displace balls in another duct for displacing a pin which controls the actuator. That second duct with balls serves also as a lock against dual key depressing.