Hi ndunia39,
We would love to help with the issue you're having, to see if we can get it resolved for you. First, we recommend checking to see if any iOS updates are available for your device, and installing any available updates.
Update your iPhone or iPad - Apple Support
If your device is updates, let's make sure you're seeing the options to decline a call as they should be on your device.
"Decline a call and send it directly to voicemail
Do one of the following:
- Press the side button twice quickly.
- Tap
. - Swipe up on the call banner.
You can also swipe down on the call banner for more options."

Reference: Answer or decline incoming calls on iPhone - Apple Support
If you do not see the option to decline a call, please reply and let us know what the options you're seeing for an incoming call show as on your device. We'll be glad to assist further!
Cheers!