iPhone gives all incoming calls busy signal
iPhone XR can make calls but gives all incoming calls a busy signal. Do not disturb is off. Call forwarding is off. The Apple checklist for the issue has been followed.
iPhone XR, iOS 13
How do you know it gives busy signal or blocking them?
Have you checked Silence Unknown Callers
To turn on Silence Unknown Callers, go to Settings > Phone, scroll down, then select Silence Unknown Callers. Calls from unknown numbers are silenced and sent to your voicemail, and appear in your recent calls list.