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Explanation: /b2 s CORRECT,OFFICIALLY APPROVED
The custom calling service that provides a tone burst to a customer on an established call when a second call has been directed to that line. The notification tone is heard only by the called customer, whereas the incoming caller hears regular audible ringing. The customer can place the existing call on hold, connect to the calling party, and then repeat the procedure to reestablish the original connection. (ENBELL, 80: 636) Custom calling services: A group of four services provided by Electronic Switching Systems to business and residence customers: 3-way-calling, speed calling, call forwarding and call waiting. (ENBELL, 80: 644)