NTT Docomo, the nation's largest mobile carrier, traced the failure to a malfunction that occurred when attempting to switch to a new computer network storing location data of phone users. That resulted in data being rerouted to the old computer network and led to a re-registration of location data from phones around the world. The huge volume of activity flooding the computer network in turn disrupted mobile phone calls and internet email services. Even after the computer network problem was put right, it took several hours to overcome the concentration of phone calls and data transmissions. Normal service for most mobile phones was not restored until around 5 a.m. on Oct. 15. However, users of the older mobile phone models continued to face problems. As of the end of June, about 14 million subscribers relied on 3G mobile phones. The company has about 80 million contracts that use Docomo transmission lines, including customers of discount mobile phone operators who lease their lines from Docomo.