There is a product we sell on this site called Westbay Traffic Calculators. Although it is not currently advertised, it includes an Add-in for Excel which adds two functions to any Excel workbook.
One is ErlcAgents() which estimates the number of agents required, and the other is ErlcCalls() which estimates the number of calls which can be handled.
If you would like any more information, please email firstname.lastname@example.org
Ansapoint Call Center Software.