Can you please follow this steps:
To troubleshoot FaceTime issues, please follow these steps:

Confirm FaceTime is Enabled: Go to Settings > FaceTime and ensure the toggle is turned on.

Check Cellular Data for FaceTime: Navigate to Settings > Cellular > Use Cellular Data For and confirm the slider next to FaceTime is turned on.

Network Connection: FaceTime calls require a 3G or faster wireless connection or a Wi-Fi connection. Ensure you have a stable internet connection.