I have a 9408 set programmed as a 2420 on CM version 6.0. Per the documentation the 9408 can have up to 3 BM12 modules attached. When you program this set as a 2420 you only get the option yes or no for expansion module. How can I add multiple modules? If I program as a 1416 I get the option to say how many modules but I lose some of my buttons on the phone.