A usage charge once per day might make sense.
It should then match the (previous) days volume?

So far amounts from Call History:

02/03/10 $0.022
02/02/10 $0.217
02/01/10 $0.000
01/31/10 $0.000
01/29/10 $0.238
...

Not sure how the are breaking down $3.06

Maybe if it said "Usage Charges DD/MM/YY - DD/MM/YY: $X.XXX"