Microsoft is investigating reports that smartphones running its Windows Phone 7 operating system are transmitting huge chunks of data over 3G networks that cause subscribers to exceed their monthly allotments.

The phantom data can be as much as 50 MB and is often consumed at the same time on consecutive days, according to one query posted to Paul Thurrott's Supersite for Windows. The reader reported that 30 MB to 50 MB of data was sent at 10:41 p.m. each day from November 21 to November 24. From December 1 to December 4, a similar amount was sent at 9:41 a.m. After the user briefly disabled 3G on December 23, the phone inexplicably sent 400 MB.

“One has to wonder when/if this trend is going to level out,” Thurrott, who is experiencing the same glitch wrote. “But it’s pretty clear now that AT&T’s lower-end 200 MB data plan would never cut it, for me at least, unless I was interested in really micro-managing usage.”

People posting to other forums report much the same thing. A reader named SoN][c said data usage is around 2 MB to 5 MB per hour, as measured by Mytelusmobility.com, even when the handset is idle.

“We are investigating this issue to determine the root cause and will update with information and guidance as it becomes available,” a Microsoft representative told the BBC, which reported on the glitch earlier. ®