When the call-waiting feature is activated, the active call can be placed on hold while another incoming call can be picked up. After finishing you can return to the first call.
Moreover, it is possible to place the active call on hold, dial another number and after talking come back to the first call. Please note that if the feature is active and you place the call on hold, both calls will be billed: the one that is active and the one that is placed on hold.
Call-Waiting can be disabled in your mobile handset, please refer to your phone’s manual.