Unconditional forwarding

I work in my company CS and all my clients have my direct number - I need to unconditionally forward calls after hours to our 800 support number which is monitored after hours but when I attempt to do so I get the following error: Forwarding phone number cannot be the same as your account number or any additional numbers. Please go to Call Handling and Forwarding and remove the number (866) XXX-XXX (deleted number for privacy)