Over-charged on Calls

[#1]

I'm not responsible for the line rental or call costs in my household, I just pay for the Internet connection.  Each month I print out the itemized bill and then get reimbursed.
Last month's itemized bill adds up to £3.87, but I have been charged £5.01 for the cost of the calls.  Unless I can provide details I'm going to have to take the hit myself - which I don't fancy doing every month.
Could someone please explain what's gone wrong here?

[#2]

Hi there, it looks like even though the billing period for your account begins on the 6th you made a call around midnight on the 5th and it looks like that made it on to the same bill but on your itemised phone billing it shows as the next month.
I'll get this confirmed for you but I believe that to be the case.
Adam

[#3]

Our itemised bills include a midnight cut off to determine whether or not a call should fall into that period. However, there's a small time after midnight but before billing runs where calls can be processed but fall into the next itemised period for display purposes. We're working on a major change which will resolve this issue and provide better bill formats, etc. unfortunately I can't provide any further information about it at this stage. Please rest assured that the total you have been charged is correct; you can look at the next itemised billing period to find the details of the call.

[#4]

OK, thanks for the explanation guys.  I'll wait to see what appears on the next bill.