China United Telecommunications (China Unicom), the country's second-largest mobile operator, has begun tests of a service that combines mobile phone service with a VoIP (voice over Internet Protocol) service, according to the U.S.-based company that supplied the technology for the project.

China Unicom is running the trial service in China's southern Guangdong provinces, said BridgePort Networks in a statement. The service allows subscribers to use the same phone number to receive VoIP calls over a broadband network when they are at home and to receive calls on their mobile phone when they are out, it said.

BridgePort did not offer specific details of the trial service, including the number of subscribers that are participating or how long the tests will last.

China Unicom's Guangdong subsidiary could not immediately be reached for comment.

VoIP is an increasingly sensitive area in China, where concerns have arisen over the technology's effect on the revenue of fixed-line operators. These concerns came to a head last week in Guangdong, where China Telecommunications, one of the country's two major fixed-line operators, cut off access to Skype Technologies' SkypeOut service in Guangdong.

According to government regulations, VoIP services that connect to the phone network can only be offered by licensed operators. That effectively blocks other companies from making available services like SkypeOut, which allows users to place VoIP calls from a computer to a phone anywhere in the world at a fraction of international calling rates.