During a call, there is a row of on-screen buttons towards the bottom of the screen. These give access to the in-call features of your phone. From left to right, they are:
1) Keypad: This is needed when navigating audio menus or dialling in an extension number, after having connected a call. As you enter more numbers, they will be appended to the string of numbers that you already dialled;
2) Loudspeaker: Useful for when you need both of your hands free and for when you want others to hear and/or participate in your call;
3) Mute: If you need to speak privately without the caller hearing you, this button will disable the phone’s microphone;
4) Hold: This will place your call on hold and allow you to make another call without terminating the current call;
5) Add contact: Tapping this will take you to the People app where you can choose an existing contact, or create a new one, in which to save the number of the current phone call.