British Telecom is working with police to find out what happened after a suspected phone scam left six households on a street with bills of around £250 each.
People living in Pelham Road, South Woodford, received bills, on the morning of April 19, including calls to places a far afield as Guinea Bissau and Bosnia and Herzegovina.
A spokesman for BT confirmed it was working with police to identify what had happened and advised customers to contact them if they see any irregularities in their bills.
He said: “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."
Brian Phipps, 67, received a bill of around £200 from British Telecom, and is worried it could happen again.
He said: “We have received no guarantee that we will not see a repeat of this so it is worrying.
"I was not alerted to it happening, I was lucky we have a network here and [neighbour] Nathan contacted me.
“I checked my bills and couldn’t believe it, I just wondered what was going on really.
"I was pretty sure I wouldn’t have to pay the money as it was clearly not me."