On #iOS Cutting calls can be rude to people and so always make use of the reply with text feature built into iOS whenever you just cannot pick the phone. iOS allows you to set 3 presets and you can also type in a custom message quickly from the lock-screen.
By default, you'll get three pre-written options ("Sorry, I can't talk right now", "I'm on my way" and "Can I call you later?"),
You can type your on message presets by tapping on the options found in:
Go to Settings>Phone>Respond With Text