Don’t rely on yourself to remember to return calls later – quickly send a text response, or set a reminder using the Remind Me or Message buttons on the call screen.
Remind Me lets you set up a reminder for later. Or choose Message to send a default text.
To change the default message options, open the Settings app, select Phone, then Respond with Text. Tap on any of the default entries to edit the text. This new phrase will then appear when you choose the Message option in the future.