A long-known bug can still freeze two series of cell phones manufactured by Siemens AG if users haven't yet installed a patch.Siemens' 35 and 45 series phones are affected by the bug, said Siemens spokesman Michael Stenberg. The company's new 55 series is not affected.The bug, according to Stenberg, is linked to the enhanced messaging service (EMS) technology used in the phones. By sending a single word, for instance, from the phone's language menu surrounded by quote marks and preceded by an asterisk as control code to an EMS-capable handset, people can cause the phone receiving the message to freeze.EMS software contains symbols, such as hearts, which are activated by short text messages containing control code. The flaw in the Siemens' phones, according to Stenberg, allows a corrupt message to tell the devices to activate a symbol that doesn't exist. The search for nonexistent symbols freezes up the handsets, he said."The bug in the 35 model surfaced shortly after the handset went into production over two years ago," he said. "We immediately provided a software update, which users could obtain from our service points." The series, distributed in Europe and Asia, is no longer in production.A software update is also available for the 45 series, launched over a year ago and still in production, Stenberg said. The newer 45 series and the latest 55 series are available in Europe, Asia and Latin America.Siemens opted not to inform all handset customers of the problem, fearing that such a move could prompt misuse by certain groups, he said. The company's hotline and service point teams have provided and continue to provide information and software updates upon request, he said.The bug causes no problems other than freezing up the handset, Stenberg said. "If you're aware of the bug and see a corrupt message, you can simply delete it," he said. "And if you happen to open it and your handset freezes, all you have to do is turn your phone off and then on again."Stenberg declined to disclose the number of handsets that could be affected by the bug and played down the significance of the technical flaw.Siemens isn't alone in combating bugs in phones that provide advanced messaging features. Nokia also had to confront a bug that could make its 6210 cell phone vulnerable to a denial-of-service attack, similar to the types of attacks that are commonly launched against computer networks.