While your phone is idle (on the hook), dial 9398, 6, then #.  You can try 1-9 to see what the different tones are and press # to save them.  Example, after you dial 9398, press 1.  If you like that one, press #.  If you don't like it, press 2.  Again, if you like that one, press # to save it.  If you don't like it, press 3 then # if you like that one.  Continue until you find one you like and press # to save it.  

Any questions, please open a helpdesk ticket with IT