In most cases, you should be able to set up a speed dial for it.
From a superconsole select:
FUNCTION
ATT FUNCTION
ABBR DIAL
It will then ask you for an "index number". That will be the speed dial number you assign to the cel phone.
Enter in a valid index (between 1-999).
Then press F2 for ENTER
You will then be prompted for the number that you want to dial.
Do not forget to put the leading digit that you use to get an outside line EG: 8 or 9 etc.
So if the cel number was 123-456-7890 and your leading digit to get an outside line is 9 you would enter in the field:
9123-456-7890
After keying in the number, select F2 for SET.
Then choose exit.
Verify your speed dial by dialing the leading digits to access the speed (abbreviated) call and then the index number you chose.
If it works, then from the superconsole, find the extension you want to forward to the cel.
Select the call forward option (Busy, No Answer, or Always) then set the call forward to the speed dial you just created.
Hope that helps