Microsoft has told BBC News that it is investigating why some handsets running its Windows Phone 7 software are sending and receiving "phantom data".
Several net forums detail complaints from people that say their phones are automatically eating into their monthly data plans without their knowledge.
Some have complained that their phone sends "between 30 and 50MB of data" every day; an amount that would devour a 1GB allowance in 20 days.
Most complainants are based in the US.
"I received an e-mail from AT&T saying that I was close to my 2GB data limit which truly shocked me as I feel I do not use data that much," a phone owner called Julie told Paul Thurrott's supersite for Windows.
"I went and looked at my AT&T account online and noticed that my phone was sending huge chunks of data seemingly in patterns."
Another said that they had noticed that the phone's "idle data usage is around 2-5MB per hour".
"This seems quite excessive to me - what exactly is being transferred? This even happens at night when I'm not getting any e-mails at all," they wrote on Howard Forums.
Some have speculated the problem may be related to the phones sending "feedback" to Microsoft about the software's performance or that the phones are using a 3G connection even when wi-fi is available.
"We are investigating this issue to determine the root cause and will update with information and guidance as it becomes available," said a Microsoft spokesperson.
Windows Phone 7 was launched in October 2010 to acclaim by manufacturers and users.
It is considered the company's first credible challenge to rival operating systems from Apple, Google, Research in Motion and Nokia.
At the recent Consumer Electronics Show in Las Vegas, Microsoft boss Steve Ballmer said that the firm would soon release "a series of platform improvements" in response to users' feedback.
These will include copy and paste capabilities and performance improvements when loading or switching between applications.