Refuse Calls - missing button
Solution 1:
Decline a call and send it directly to voicemail
Press the […] Sleep/Wake button […] twice quickly.
https://support.apple.com/guide/iphone/iph3c9947bf/ios
Solution 2:
If your phone is locked, only slide to answer is present. You have to click the lock button once to silence, and twice to decline it. If it's unlocked, you can decline it normally.
More details here