When you are on a phone call and someone calls you, you will be alerted with an audio (i.e. ringtone) and visual alert (i.e. blinking screen) of the incoming call. You can answer the second call while keeping your first call active by pressing the  key on your phone or by following the instructions displayed on your phone. At this moment, you will be able to switch back and forth between the two calls in progress. Please refer to your Phone manual for more details.