the least favorable aspects of iOS, and even seasoned iPhone users are often unaware that it serves as a feature benefiting them. The Accept and Decline choices only emerge when the iPhone is unlocked. If it’s locked, you can merely slide to respond.
The fundamental explanation for this is to avoid unintentional actions, whether it’s answering or rejecting a call. When your iPhone is locked, it’s presumed that you are not actively engaged with the screen. It may be in your pocket, your bag, or situated on a table right before you.
Now picture if the Accept and Decline options were visible on the lock screen and your iPhone rang. You could accidentally tap one of them while taking the phone out of your pocket or lifting it from the table. To prevent this, Apple substituted those choices on the lock screen with the “slide to answer” feature. With this setup, even a few unintended touches on the screen will have no effect. It will neither accept nor decline the call, but you can still reject calls from the lock screen.