I don't know if you got your answer yet. I have DTU-32D-2 (attendent phone) and the extension phones in the office are DTU-16D-2 phones (Electra Elite-48 system). I transfer incoming calls to outside lines quite regularly (including cell phones). Most importantly, you need an available outside line - your incoming call takes up one line, but it will need to "exit" via another outside line. The simple way to transfer a call to the "outside" is to hit [Transfer] then dial [9](for the outside line, assuming that is how you are set up) and then the telephone number - the line will ring and once you are connected to your intended party, then hit [Feature][Transfer] to connect the call. Once you do this, the line lights will switch from green to red and you will no longer have access to the call. If the cell phone or other external party is a frequently used number, you may have it set up as a "speed dial" on your [Sys.] or [Sta.] softkeys - in which case, this number will have a speed dial number associated with it (just hit your speed dial list and scroll through it - note the number that appears beside your desired contact). To use this, when transfering the call to an outside line (the cell phone) hit [Transfer][Redial] and the two digit speed dial number (if it does not include a number to dial an outside line, include that before the number) and then, once you have connected with your other party, hit [Feature][Transfer]. This works for all of our phones (although less technical users still like to call me when they need to make such a transfer).
Good luck. Lorraine