BT says it has acted to stop criminals tampering with the phone network after customers in Norfolk were sent massive bills for calls they had not made.
One bill showed £1,300 of high cost calls made in just three hours
In one area, junction boxes had been interfered with and elsewhere, internet users ran up big bills after activating premium rate lines inadvertently.
An investigation has shown calls were not made from the customers' homes.
BT says it is refunding those affected and investigating the situation to identify the perpetrators.
For John Uff, who runs an internet fax business, his bill was a bombshell, with £1,300 of high cost calls made in just three hours.
Mr Uff said the thief had made 10 different calls at the same time in the early hours of the morning.
It was later discovered that phone lines had been stripped back to the copper wire, allowing someone to tap in and dial up premium rate sex lines. The police are now investigating.
At Thurne in Norfolk, Ann Lamb, is among the 28 villagers who had calls made from their numbers - all between midnight and 0300 BST - at £1.50 a minute.
"We realised that there were three premium rate calls that had taken place in the middle of the night and we didn't make those calls," she said.
Fellow villager Pat George was in Portsmouth when calls totalling £60 were dialled from her number.
At nearby North Walsham internet hackers programmed retired head teacher Edward Barber's computer to dial up premium rate lines at a cost of £800.
A BT spokesman said: "A number of customers in East Anglia have been affected by illicit access to the BT network, with some receiving bills with unrecognised calls.
"An investigation has shown the calls were not made from within customers' homes.
"We are refunding those affected and investigating the situation to identify the perpetrators."