Most likely: Settings → General → VPN & Device Management

If VPN shows “Connected”

This can absolutely cause your symptoms, including:

• Not receiving incoming calls

• No call records

• Signal bar fluctuating

• But data and SMS still working

Because some VPNs reroute all network traffic, including the carrier’s voice-over-LTE registration.
If that registration fails → incoming calls never reach your phone.

This is very common after installing a verification or email security app.

turn it OFF and test your calls.
Settings → General → VPN & Device Management → VPN → Disconnect

You may not even notice it turned on automatically.