When Nikki Christie rang BT to ask why there were £93 of mystery phone calls to Bosnia and Guinea Bissau on her bill – calls she knew no one in the household could possibly have made – the response, she says, was unequivocal: "There isn't a fault on your line. Someone in the home must have made them."

But Christie was amazed, and then increasingly annoyed, to find that many of her neighbours were in exactly the same position. Seven homes, it transpired, in Pelham Road, South Woodford, in north-east London, had seen in total more than £1,000 of international calls added to their phone bills – some homes were charged as much as £250. The calls were all made on the same date – 19 April – in the middle of the night, while the neighbourhood was asleep.

Despite this, BT staff insisted Christie and her neighbours were liable – despite the fact that the only possible conclusion was that someone had managed to hack into the phone system and charged the calls to their accounts.

The residents are furious because it has since emerged that while this is a well-known problem in telecoms circles, no one at BT had told the call centre workers – and as a result, residents say, they spent hours talking to customer services and getting nowhere. The fact that Sky customers in the same road, who were similarly affected, had the problem efficiently dealt with only added to their sense of frustration.

What the residents of Pelham Road did not know is that they had been victims of something known as "teeing", or "teeing-in". This sees fraudsters physically taking over a victim's phone line, by connecting a handset and using it to make calls – often to foreign premium-rate numbers.

According to a telecoms veteran, it can happen at the green boxes on street corners, or most likely, at the junction box next to the victim's home. While it's a fraud that can happen to any of us, few people are aware of it, and the telecoms industry appears to want to keep it quiet.

It was certainly a surprise to Christie, who says she has spent hours trying to resolve the problem. "BT's call centre staff have no idea what they are doing, and had I not been in a position to show that other people had suffered the same problem, I would still be waiting for a refund. I spent over an hour on the phone being bounced between the billing and faults teams. Billing could not refund the money as they said there was no fault on the line, and faults would then listen to my story.

"I spoke to at least six different operatives, and had to repeat my story to all of them. At varying points the Bosnian number was read back to me and I was asked if I recognised it. I was told several times that as there was no fault we must have made the calls, and it was suggested that we might have been out of the property and the calls were made in our absence."

She says that of the seven households she knows were affected, the five that are BT customers all reported having a terrible time getting it resolved.

It was a similar story for another victim, Vinoth Ramiah, who lives not far down the road from Christie in Ilford.

Ramiah separately contacted Money after he found £225 of international calls to Bosnia on his most recent bill. The calls were all made on 6 April and occurred every 10 minutes, for the exact same duration of eight minutes, 55 seconds, and to the same three numbers. They started just after midnight and ended before 6am, but he says the BT call centre initially refused to acknowledge any fault and he was told that he was responsible for the calls – until, finally, he came across a BT staff member who recognised the scam.

"He was very helpful, but he revealed that the BT overseas call centres are not trained to deal with this issue and so have no idea that this is a problem that can occur. In fact he only recognised it as he used to work in this team known as the 'high value accounts team' who deal with such issues."

Both Christie and Ramiah contacted Guardian Money after reading the story of a man from Leicester who found £700 worth of calls to Iraq had been made on his newly installed line – despite there not being a handset in his holiday home. At the time, BT told us that someone must have accessed the house, as it was not possible to hack a phone line, although it refunded the charges as a "gesture of goodwill".

A BT spokesman said: "The telephone lines of several Pelham Road residents were targeted by a criminal gang making fraudulent calls to international numbers. As a result of an investigation by the police and BT, three people were arrested and have been released on police bail pending further inquiries.

"We try to prevent customers being billed for fraudulent calls, but we may not be able to do this when the billing process is already underway. Where fraud is identified we try to prevent calls to the numbers concerned. We have credited the accounts of the customers affected. We apologise to customers who have had difficulties in resolving the issue with our advisers."

"Scams and incidents like this are taken extremely seriously and we work extensively with our industry partners and the police to eliminate them. We will continue to work with the police on this particular case so that the people responsible are brought to justice. Our advice to any customer who has unexpected charges on their bill from numbers that they have not called, is to contact their phone supplier to report what has happened."