Here is another completely free source of the Erlang formulas for Excel (called ErlangXLS):
http://www.cclogic.eu/download/en/Er...lish_Setup.exe
It adds Erlang C and Erlang B to your Excel functions, so you can use them in your own spreadsheets to calculate Service Level, ASA, agent occupation, trunk load, etc.
There is more information on the provider's homepage:
CC-Logic: Call Center Software | Call Center Planning and Optimization