Just as speech is silver but silence golden, it can cost more to block a mobile phone than it does to buy one. Devices to jam phone signals cost up to $50,000, and even the cheapest are about $1,500. According to the Office of the Telecommunications Authority, jammers work by transmitting a signal in the same frequency bands as mobile phones. The jamming signals interfere with the normal functioning of the mobile phone, meaning users in the vicinity of a jammer are not able to make or receive calls. Preventing 'leakage' - the accidental blocking of signals outside the intended area - required careful technical planning, the spokeswoman said. Improper installation could cause interference to users of mobile phones more than a kilometre away, she said. A Canadian Government consultation paper on the issue mentioned a range of different types of blockers, one of which could instruct compatible phones to disable their ringers and vibrate instead. Feedback on the issue in Canada has been voluminous - including questions of legal liability if fire or ambulance calls were affected, and fears over the possible blocking of emergency calls or pages to doctors. But associate professor of electrical and electronic engineering at the Hong Kong University of Science and Technology Dr Ross Murch said medical pagers operated on a different frequency to those used by mobile telephones. It would be possible to enable pagers to operate while disabling mobiles, he said, a view echoed by the telecommunications authority. In Canada, a poll reportedly found 68 per cent of the population favoured jamming mobiles in places of worship and movie theatres, 65 per cent in public libraries and 57 per cent in hospitals, planes and galleries. But 78 per cent of those polled opposed jamming mobiles in shopping centres.
