Hong Kong's telecoms watchdog says it has solved the riddle of why some of the city's mobile phones switch to mainland networks even when used far from the border. An investigation by the Office of the Telecommunications Authority (Ofta) has studied mainland signals that have been causing mobile phones on Hong Kong Island to switch to the more expensive roaming service. It found the signals originated in communities on the Wanshan Islands, off the southwestern coast of Lantau Island. The revelation follows Sunday Morning Post reports that mobile- phone users on Hong Kong Island had been caught unawares by large bills after their phones switched to the more expensive roaming service. A list of affected areas on Hong Kong Island included Pokfulam, Mount Davis, South Horizons and Chung Hom Kok. Ofta had previously maintained that the phenomenon occurred only in rare cases along Hong Kong's border with the mainland. The probe found the interfering signals originated on Guishan Island, the largest of the Wanshan Islands, and from the nearby tourism centre of Wailingding Island. Mobile-phone radio waves can be blocked by terrain and buildings, but can travel far without losing signal quality, over open water. However, Ofta denied that the problem was widespread. '[The signals] can only be detected at high levels of some buildings, and the local coverage is overall acceptable,' a spokeswoman said. Phones in only some of the households in the affected areas accidentally switched to mainland networks, she said. Mobile-phone signals from the Wanshan Islands should be easier to deal with than those coming from Shenzhen, which had plagued users living close to the border, she added. Ofta said it had passed the findings from the probe to the Ministry of Information Industry - its mainland counterpart. The two sides met twice in the past month to discuss the issue, but so far no plans to tackle the problem have been agreed. Ofta said it would be fixed 'in due course'.