Why Slide to unlock doesn't always show up on the iPhone?

This question has been asked for years by iPhone owners: Why can't I stop calling without opening the phone?

Instead, the only option offered is “Slide to unlock”, allowing users to ignore calls and just wait until the call stops.

Sometimes it shows up, but often not many users have reported on it on social networks, having no idea why this happens.

If iPhone screens are locked, “will be shown to answer”, but if it is open, the button to answer and stop calls appears, as in the photo below.

Many users don't know how to stop calling, but there's a trick to do it.

If you push the button that opens the iPhone, the roar and the vibration stop, but if you press it twice then stop calling.