*72 call forwarding not working - does not take the area code first - and dials the wrong number

I dial *72 to call forward on ATT phone to a T-mobile cell phone.

ATT does not take the full 10 digits with area code first. it dials only a 7 digit number. it does not accept 10 digits with area code.

I called tech support and they say it's a problem with Tmobile but it's not because the number that gets call fowarded is the first 7 digits of my 10 digit cell number. A guy always picks up with that number. Please HELP!! TECH support not helpful. Thanks!