The culprit behind some mysteriously high cellular data usage by Windows Phone 7 devices has been attributed to a third-party service, and not necessarily the software OS itself, Microsoft said today.

The "phantom data" problem, which has left some users burning through their monthly cellular data allotment in short order (even when they were connected to Wi-Fi), was addressed by the software giant last week. Microsoft pledged that it would begin an investigation into the matter, though had not yet provided an update.

Speaking to the Seattle Post Intelligencer, a Microsoft representative said the company had figured out what it believed to be the cause of the heavy data downloads, which it attributed to an unnamed third party. The company also said that it was at work on ways to fix the problem, which had affected only a "small" percentage of Windows Phone customers:

"We have determined that a third-party solution commonly accessed from Windows Phones is configured in a manner that potentially causes larger than expected data downloads. We are in contact with the third party to assist them in making the necessary fixes, and are also pursuing potential workarounds to address the configuration issue in case those are needed. At this point in our investigation, we believe this is responsible for most of the reported incidents.

We are investigating additional potential root causes for the remainder of the reports.

A small (low single-digit) percentage of Windows Phone customers have reported being affected.

We are continuing to investigate this issue and will update with additional information and guidance as it becomes available."

Assuming Microsoft can work out the leaky data issue with the unnamed third-party, the problem could be fixed without the need for a system software update. The first update for Windows Phone 7 is headed to carriers for testing later this month.