Hi,

I think the most versatile and powerful tool for this task would be to use pivot tables. From a very simple table of start and finish times you can easily calculate a number of different things. See attached workbook...

.xlsx version
CallCentrePivots.xlsx

.xls version (only try this if above doesn't work)
CallCentrePivots.xls

If you've not used pivot tables before you might have to do a little bit of reading but they are perfect for these kinds of calculations.

Hope that helps

Pete