Absar, ErlangB or Hills B is the way to go. Hills B is quite expensive, but if you put ErlangB into an internet search engine you will be able to find loads of free Erlang Calculators and some add-ins for excel.
Happy hunting
Absar, it depends on the type of business you are looking at. If it is Technical Support, then the cap is 3 calls per hour per agent or 30 calls per agent per day (8 hours shift).