To answer you question, MitelTech is very correct in the portion regarding how the CallerID (CLID/ANI) is sent to the phone system.
Regarding how the information is sent to the display, it can be changed by the manufacture. When a phone call comes into a phone, your phone system (Mitel or Nortel) "talks" to the phone first. It sends it information like what button to light, what information to display on the display, and commands like "RING_PHONE" and "RING_PHONE_STOP." The phone is actually a very stupid computer. It can't funcition without the switch (the PBX) their to give it instruction. Think about your printer, it does it's job well, prints when it's told to. Take your computer away and it's useless.
The display information on your phones display is simply how the manufacture wanted it displayed and it CAN be changed to be formated to look better. With that said, it is the manufacture who needs to make they're system display things properly.
As for the portion where the CLASS is discussed, I'm not so sure that's true. When I plug in a VISTA 350 to my 1FL I get "(xxx)xxx-xxxx." When I plug in my SX-200 ICP MX, on the same 1FL, I get "xxxxxxxxxx." With that said, I wonder if its the phone/phone system that does the formatting???? (more then likely).
Neil