I would like to add one thing to Erlang’s answer. I agree with his upper and lower bounds. I would add that when we use Erlang-B to provision a trunk (or whatever) we are estimating the number of simultaneous calls. For example, if we have 15 Erlangs in the busy hour and 1% blocking we would provision 24 servers (i.e. DS0s). In essence this is saying that 1% of the time we have 24 simultaneous users (maybe more that are blocked.)
That is one way to estimate the number of simultaneous calls (assuming you can use Erlang-B). There are other tools you could use to calculate the number if you had a system that you could take measurements on as well (but I think that is another discussion.)
Let me know what you think and feel free to send me an email (email@example.com) if you want to discuss this in more detail.