The way to answer calls is different depending on type of device, OS version, condition of device or settings.

For iOS 11 or later (and when the Integrated Calling setting is disabled.)

Receive Calls on the Lock Screen

In order to answer a call, slide to the right.
Note: If you set a passcode on the lock screen, you must unlock it.

If you can't answer a call, pop (press deeper) the notification.

Receive Calls on the Home Screen

Tap a banner to answer calls.

If you can't answer a call, slide a banner down or pop (press deeper.)

For Android 8.0 or later

Notification setting: Show Banners Only

Select "Answer" or "Decline."

Notification setting: Show the Calling Screen

Select "Answer" or "Decline."