Users are reporting that phones using Microsoft's WP 7 operating system are uploading mysterious packets of data, and costing money in the process.

It isn't clear what the 'phantom' data is. But at several online forums for Windows Phone users reports are coming in that the phones are showing massive amounts of data usage on users' bills.

One letter came in to Paul Thurott, who runs the Supersite for Windows. A person named Julie G. wrote: On December 22nd I received an email from AT&T saying that I was close to my 2GB data limit... I went and looked at my AT&T account online and noticed that my phone was sending huge chunks of data seemingly in patterns. For instance on November 21-24 it sent between 30 and 50 MB of data at 10:41 p.m. each day and Dec 1-4 it sent between 30 and 50 MB of data at 9:41 a.m. each day. AT&T failed to tell her what the problem was.

Thurott has been tracking his own usage on his blog, and noted that data usage with the Windows phone he has went up significantly since he purchased it in July, though it seemed to drop somewhat in December.

Other users have also noticed the problem. On Howard Forums, a user, Son][c, described his Windows phone using 2-2.5 megabytes per hour.

It isn't clear what the problem is. Those who have posted about the issue ascribe the data usage to the phone using the 3G network connection rather than Wi-Fi to transmit - even when it is in range of a Wi-Fi network.

A Microsoft spokesperson emaild a statement that said, We are investigating this issue to determine the root cause and will update with information and guidance as it becomes available