Do you see a lot of mobile calls?
The Mobile Call Breakdown Report will give a summary of mobile dialling patterns.
If this is a bit higher than you would expect it may be because some extensions have been diverted to a mobile. If so, you will be paying for incoming calls to be transferred out again. You can see this by running a Party Report filtered for incoming calls only, then see if there are any dialled numbers.
It could be that staff work away from their desk and you could save money by using DECT phones instead.