Microsoft has told BBC News that it is investigating why some handsets running its Windows Phone 7 software are sending and receiving "phantom data". Some have complained that their phone sends "between 30 and 50MB of data" every day; an amount that would eat into 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.
"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".
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're investigating this issue to determine the root cause and will update with information and guidance as it becomes available," said a Microsoft spokesperson.