Microsoft is looking into reports that phones running Windows Phone 7 are sending out large amounts of data without their users' knowledge.
Users say they've been waking up to find that their phone has been chatting away all night without them, often pushing them over their monthly data limit.
"When I am home I have the phone logged into my Wifi network. I noticed that my data usage seemed high, so I figured I would track it," says one user on a company support forum.
"On Wednesday at 1:48 pm I had used 33.58MB, today at 9:30 I had used 35.41MB. That is almost 2MB in less than 24 hours. The phone never left my house and was logged into the Wifi network the whole time."
Other users are suggesting that the data transfer may be related to the phones' sending feedback information to Microsoft. It could be the case that the phone is using a 3G connection even when Wifi is available and enabled as the default network.
Turning off data altogether works - but only if users are prepared to keep it off indefinitely.
"On December 23rd I turned on airplane mode so my phone could no longer send data. I turned airplane mode off briefly on December 23rd and the phone sent 400 MB of data," says a user on the Supersite for Windows forum.
"I called AT&T yesterday, December 28th, but they said that there was nothing that they can do to figure out what was happening on my phone."
Microsoft's told the BBC that it plans to investigate what's going on, and will update users when it's worked it out. [[Microsoft]]