Call forwarding command not recognized

[MDinFL]

I am on ATT in the US.  The USSD command for invoking call forwarding is:

1. Dial **21*, followed by the 10-digit number you wish to forward the calls to, then dial #. For example, **21*1235556789# forwards your call to 123.555.6789.
2. Push the Call, Send, Talk, or Receiver icon.
3. Your wireless phone displays a confirmation message when Call Forwarding activates.

Turn off Call Forwarding
To turn off Call Forwarding and stop sending incoming calls to another number:

1. Dial #21#.
2. Push the Call, Send, Talk, or Receiver icon. 
3. Your wireless device displays a confirmation message when Call Forwarding deactivates.

I followed these instructions and get an error message "Connection problem or invalid MMI Code"

There is a variety of webpages all with the same solutions, none of which work.  I also tried using the Call Forwarding function in the OS.  Same problem.

[MDinFL · Accepted solution]

For about 6 days, I had a Samsung Galaxy S7.  It sucked and I returned it.  I put the SIM from the Samsung back into the Sony.  Apparently, I needed to contact AT&T to register the SIM card to the Sony IMEI.  Once that happened, the forwarding command worked.