Japan's major telecommunication carriers have revealed a worrying rise in the number of hacking attacks against IP phone systems in which users, mainly companies, are being charged large amounts of money for overseas phone calls they didn't make.

Major IP phone service providers NTT East Corp. and NTT West Corp. said that in the 2014 business year they received 101 complaints from IP phone users claiming they had been charged for international calls they were not aware of.

In one case, a user was charged with a monthly fee of more than ¥5 million, they said.