Microsoft has pointed the finger at a "third-party solution" for unexplained Windows Phone 7 (WP7) data charges after receiving complaints from users.
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Internet forums were flooded with complaints about unidentified data use charges on WP7 devices, with some users wrongly charged for up to 50MB a day for checking e-mail and social networking sites.
In a statement, Microsoft said, "We have determined that a third-party solution commonly accessed from Windows Phones is configured in a manner that may potentially cause larger than expected data downloads."
"At this point in our investigation we believe this is responsible for most of the reported incidents," continued the statement.
The company said necessary fixes were being made, while workarounds to address the configuration issue were being pursued.
Microsoft confirmed that a "small percentage" of Windows Phone customers had been affected. The firm will continue to investigate complaints and potential root causes.
Microsoft launched Windows Phone 7 in October 2010 for nine different handsets. The company recently admitted it was only "comfortable" with WP7's progress after phone manufacturer sales reached 1.5 million.
The first of several updates to WP7 operating system is expected in the coming weeks.