When receiving a call, you usually have to swipe from the centre of a circle to a red or green icon to either reject or accept a call, respectively. This is similar to how you can unlock your device’s screen.
You may not have noticed, but there is also a message icon at the top of the circle. Swiping to that icon will reject the call and give you a selection of messages to acknowledge the rejected call. Select the most appropriate and it will then be sent to the number that called you. These messages are called ‘quick responses’ and they can be changed from the Phone settings screen.