During a call, you can use some buttons near the bottom of the screen
Switch to the dialer.
Control the audio output
Mute your microphone
Put the other party on hold
Switch to another call in process
Add another call
Combine two or more calls
If there is another call coming in, you will get a notice on the screen.
You can answer it directly which will place the other call on hold.
You can touch to switch to another call in process.
Note: please make sure you have the three way calling service with your SIM card.