Microsoft has said that an unnamed third party service is causing most "phantom" data exchange problems in handsets running on Windows Phone 7 software.
The software maker said investigation is on and it is still looking into other potential faults.
Users had complained on net forums that monthly data plans in their phones were being used without their knowledge.
Most complainants were from the US.
A spokesperson said the company was in contact with the third party to assist them in fixing the problem which seemed to only affect a small percentage of Windows Phone customers.
"We are continuing to investigate this issue and will update with additional information and guidance as it becomes available," the spokesperson said.
Considered to be a rival to operating systems from Apple, Google, Research in Motion and Nokia, Windows Phone 7 was launched in October 2010.
