There is a free online calculator on this Web site which will go some way to meeting your requirements. It is at http://www.erlang.com/calculator/?calc=call.
There is also a Windows version called Ansapoint which is available for immediate download. There is more information at http://www.erlang.com/ansapoint.html.
The online version is limited to an 8 hour day, which 60 minute analytical intervals. The Windows version support a full 24 hours analysis, with 30 minute intervals. Neither solution will support 15 minute intervals.
If you have any further questions, please write to us at firstname.lastname@example.org.