I think you missed a step.. you can’t directly assume that 5000 calls = 5000 customers.
Normally, you work with erlangs, and then should use an Erlang B law translation:
5000 subscribers will perform 2 calls of 120seconds each during the busy hour.
This is equivalent to 5000 x 120 x 2 = 1 200 000 seconds = 20.000 erlangs.
Well, it becomes more complicated after that, that’s just a start.