It irritates me that people don’t understand how phone calls work on iOS even after 16 years of this feature.
1. If you receive a phone call while your iPhone is unlocked you will get two options: Answer or decline
2. If you receive a phone call while your iPhone is unlocked you will only get one option on the screen which is to slide to accept.
Because the phone is locked and is likely in your pocket or out of reach on a table there is a risk of accidental inputs or “butt dialling“.
In 2010, to fix this issue Apple switched to a “slide to answer” UI when the phone is locked.
To decline the call you double press the side button, or swipe home and wait for the call to go to voicemail.