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
If you can't answer a call, pop (press deeper) the notification.
Receive Calls on the Home Screen
If you can't answer a call, slide a banner down or pop (press deeper.)
In Android 8.0 or later