The hacker also disabled the House of Commons telephone system, the Daily Mail reported.
Claire Perry, Tory MP for Devizes in Wiltshire, 117 km from London, said she found her voicemail had been changed.
"There was a general, system-wide failure overnight. But the changing of the voicemail strongly suggests that the two things were linked, and that a good deal of planning had gone into the operation," she said. She, however, did not say what the message was.
David Whitelegg, an IT security expert, said the hacker may not have been a prankster, but that he may have been testing the security of the system.
"It's worrying because it is the sort of thing a terrorist could do prior to launching an offensive. It shows our vulnerability to a cyber attack," he said.
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