I want to set up call forwarding when busy. On the call forwarding set up page, the space under "Mobile Number" is blank, there's nothing there for me to enter or select my number (I am logged into my account at the time). I can fill in the other options, but that first one is just not there. When I submit, it just keeps asking me to select a mobile number, yet not giving me any option to actually enter one. Is that feature not available on prepaids, or is it something else?