[CCIP]

The callsigns are for radio comms, and the nature of radio comms is such that they're generally addressed to someone and require a response to confirm. So, normally that would be to the flight lead.

The nature of modern operations is also such that you rarely have squadrons, let alone wings, operating all at the same time under one callsign - usually we're dealing with flights. Squadrons and wings are organizations more so than tactical units; the basic tactical units are flights, which come together in packages. Communications to flights need to be addressed to and acknowledged by flight leads. Even assuming you have a whole squadron operating, it would still have a lead. "Devil Lead" in normal usage is just short for "Devil 11", i.e. first plane of the first flight. 12 would be his wingman, 21 would be lead in the 2nd flight, and so on. If these flights are operating together, 11 is the overall lead, and 21 is still lead for the flight. If they're operating separately, obviously the communication would have to clarify which flight lead is being called. At all points in an operation, there HAS to be a lead - even if the original lead is shot down, there is standard procedure for immediately replacing him and keeping the flight under control and in communication. Likewise, in standard operation odd numbers are element leads, even numbers are wingmen - it's like a buddy system. So, you are always responsible to your lead within your unit (flight, element, etc.), and your lead is responsible for keeping communication addressed to that unit as a whole. All the communications are very standardized, so there's really nothing terribly complex about the radio callsigns - they're just a very simple and hierarchical type of label to help everyone involved in the operation know who you are and who they need to talk to/hear from.