Once you have either an active call and a held call or two held calls, you can conference both calls together.

Press the Conf softkey to place the active call on hold.

Enter the second number that you wish to call and press Send. When the second party picks up, press the Conf softkey again to join the calls together.

Break the calls apart by using the up and down buttons to select the call that will be placed on hold, and then pressing the Hold softkey.