[Q] Looking for a group bandwidth usage app topic

Hi,
I'm looking for an application, probably integrated with some sort of web based interface, to track my employees data usage. I don't want to track phones, spy into sms messages or web pages visited, but need to know the amount of data used, per device and a report compiling the whole thing. Why? The phone provider we use doesn't provide such tool (and in Brazil I'm not aware of one that does this for closed groups) and we need to make adjustments to our data plan in a way to avoid extra costs.
Thanks for you help.