How to receive call and force that call to ring indefinitely to the caller but not the receiver
What you want is mostly a carrier-specific circumstance, and less of an Android device one.  Though, some OEMs support the ability to turn off call forwarding via Call app settings:

Device > Apps > Phone > More settings > Call forwarding > Voice call.  Then, disable these three things: "Forward When Busy", "Forward When Unanswered" and "Forward When Unreached".  This option may not be available on your device.

If the above option isn't offered on your device, you will have to log onto or call AT&amp;T customer support to turn off voice mail for your account.

See: https://forums.att.com/t5/AT-T-Phone-Features/Turn-Voicemail-On-and-Off/td-p/3606949