Reports started emerging a few weeks ago which suggested that Windows Phone 7 handsets are suffering from a bug where data was being sent and received over 3G even when a WiFi connection is available.
Some users are reporting that simply having the handset check for email every 15 minutes uses a whopping 3 - 5MB of data per hour!
Microsoft has now investigated this issue and says that it's down to a "third party" service and that the company is "in contact with the third party to assist them in making the necessary fixes." Microsoft is however still looking into other potential faults.
Several insiders who wish to remain anonymous have told me that one Windows Phone 7 blamed for phantom data usage is the Yahoo! Mail service. There's no official word on this from either Microsoft or Yahoo! at the time of writing.