During a phone call you can see five icons at the bottom:
Video call – Here you can activate your front camera in order to see each other while talking to each other.
Message – Via this icon you are immediately led to the chat of your communication partner and you.
Hang up – Here you can end the phone call.
Speakers – With this icon you can activate the speaker of your smartphone.
Microphone – With the help of this icon you can mute your microphone.