(I'm sorry I missed the first email)
Callnum is part of Koji's system for keeping retries sane. If a client makes a call and gets a network error, it necessarily tell if the hub got the call or not. We would not want to retry and perform the same action twice (or ten times). Callnum keeps the client and hub in sync and ensures that the hub will raise an error rather then rerunning a duplicate call.
So, we cannot simply remove it. There are probably other ways we could solve this, but we would need to be very careful. We also need to consider older clients.
There may be ways we can make the existing situation better in some of the ways you are concerned about. However, I suspect your main concern is the difficulty of managing sessions for parallel processing.