you can use a basic erlang B calculator, and get any result you need.
What’s the congestion rate that’s allowed in yuour PABX ?
You can find several online erlang calculators, and even excel macros.
Here is one which does not provide “reverse” results. So you can just try different erlangs, until you reach the number of lines = 2000.
Erlangs – rho = 1894[input]
Grade of Service – P(c) = 0.0005 [input] (=0.05% congestion)
Capacity/Num of Lines = 2000 [output]
But anyway, you can find anything on the net, just keep looking 🙂