This service allows a subscriber to call another party during an existing call and add this party to the call, creating a three-way conversation.

To add a third party to an active call, press the flash-hook and then dial the third partys number. If the third party answers, hit the flash-hook again to add both of you to the original call, connecting all three parties.

If the third party does not answer or their line is busy, hit the flash-hook twice to rejoin the original call.

If you hang up you will receive a ringback reminding you that the original call is still on hold, and can rejoin the call by picking up the phone.
This service is known as Three Way Calling Ringback.