ussd – Unable to Deactivate “Call forwarding unconditional”

I have a REVVL4 phone. I entered USSD code *#21# and the message “Call forwarding unconditional Connection problem or invalid MMI code.”

Although I did not set the forwarding, and after online research for the USSD codes, I tried to deactivate the forwarding with the suggested codes ##21# and #21#, but I got the same message “Call forwarding unconditional Connection problem or invalid MMI code.”

How can I deactivate the “Call forwarding unconditional”

Thx