If you receive a call on your iPhone when you are busy, you may not be able to answer it. After some time passes and you are no longer busy, you may then forget to call the person back. There is a way to have the iPhone automatically set a reminder for you to return the call.
Set a Reminder
Here’s how to automatically set a reminder when you don’t answer a call.
When receiving an incoming call, tap Remind Me instead of answering the call. The call will automatically be declined and the iPhone will stop ringing.
A window will appear where you can select to be reminded to call the person back. Tap One Hour and you will receive a reminder on your iPhone in an hour to return the call.
This should help you remember to return the call.