Hello Sterx,
Thanks for choosing the Apple Support Communities. Here is a set of instructions on how to decline a call on your iPhone 13 Pro max.
Answer or decline incoming calls on iPhone - Apple Support
"Silence a call
Press the side button or Sleep/Wake button (depending on your model) or either volume button.
You can still answer a silenced call until it goes to voicemail.
Decline a call and send it directly to voicemail
Do one of the following:
- Press the side button or Sleep/Wake button (depending on your model) twice quickly.
- Tap .
- Swipe up on the call banner.
You can also swipe down on the call banner for more options.
Do any of the following:
- Tap Remind Me, then choose when you want a reminder to return the call.
- Tap Message, then choose a default reply or tap Custom.
- To create your own default replies, go to Settings > Phone > Respond with Text, then tap any default message and replace it with your own text.
Note: In some countries or regions, declined calls are disconnected without being sent to voicemail."
Cheers!