ERICSSON has admitted that mobile GSM handsets have caused interference with car radios and at least one car manufacturer has changed radio design. Mr Largs Ranqvist, president of Ericsson, was answering a report by Communications Week International which said GSM transmissions from phone handsets caused interference on hearing aids. ''Reports have come in that radios in cars and hearing aids are being disturbed when cellular phones are being used,'' he said. But he added: ''When you try to repeat this with tests in a laboratory, it's almost impossible.'' The European Hearing Instrument Manufacturers and Australia's Deafness Foundation have already demanded action, saying tests show that hearing aid users experience a humming noise when GSM handsets are being used as far away as 30 metres. It is thought that the burst signal mode in the GSM technology is responsible for the interference.