If you’re too busy to take a phone call, but want to send off a quick message saying so, iOS has a feature to let you do just that.
In fact, you can customise some of those responses. To do so, head to the Settings app, then to “General”. There, click on the “phone” option, and then choose the “reply with message” menu.
There, you can decide which messages you want to use, and customise your own.