Hello AliceNet,
Thank you for using Apple Support Communities! I understand that you would like to add Silence Unknown Callers to the Control Center on your iPhone to quickly turn this feature off and on. It is not a customizable feature that can currently be added to Control Center, and would need to be manually turned off and on under Settings > Phone:
- Use and customize Control Center on iPhone - Apple Support
- Detect and block spam phone calls - Apple Support
If you'd like to provide feedback to Apple about this feature, I invite you to do so by visiting the following link:
Feedback - iPhone - Apple
Cheers!