Sometimes you just are not able or want to take a call. iOS gives you a handy little feature to respond with a text message and decline the call. When you press message on the screen when a call comes in you are presented with a list of default pre-written responses and the option to write your own on the fly, but if those standard options are not ones you want to use it’s possible to customise the list.
Accessing the Settings for Text Responses
Go to Settings>Phone>Respond with text. Heree you can edit the defualt set of responses. You can only have three, so in order to edit or change the ones available to you, click in a slot and change the response to what you desire.