If I'm using an App, I can't answer my iPhone.

Why not, does the phone screen not show up? Try a forced restart.

Try a forced restart.

• Press and quickly release the volume up button

• Press and quickly release the volume down button

• Press and hold the side button.

• When the Apple logo appears, release the button.

Force restart iPhone - Apple Support (IN)

If you are experiencing any difficulty with third-party apps or a specific website then please contact the app developer or site publisher/host directly --> How to contact an app developer - Apple Support

It's a little different than just turning the phone off and on, follow the instructions I provided.