Police have issued an alert after 30 people were targeted by a sophisticated fraud gang.
Detectives said the gang is posing as bank workers or police officers to try to obtain their victims' account details, enabling them to withdraw cash.
Of the 30 people targeted since the scam came to light last month, five have lost money from their accounts, including one person who had £6,000 illegally withdrawn.
The victims have all received phone calls from the criminals who claimed to be a member of staff at their bank or a police officer.
The callers claimed they had discovered irregularities in the targets' bank accounts.
They then told the victims to hang up and phone their bank.
However, the criminals have found a way of manipulating the telephone system which means the victims are reconnected to them.
Posing as bank officials, they ask their victims for their account numbers and PINs and then tell them they are sending a taxi to their home to collect their bank cards.
Leicestershire Police has confirmed it is investigating 30 cases linked to the gang. Most of the victims became suspicious and did not fall for the con.
However, five people have lost money with sums ranging from a few hundred pounds to £6,000.
Paul Wenlock, head of Leicestershire Police's economic crime unit, said: "This fraud has been around in London since 2011, but all the Leicestershire cases have been since August this year.
"We are pursuing several lines of inquiry to catch those responsible.
"However, members of the public should be on their guard.
"Banks will never ask you for your passwords.
"They will never call at your home, and the police and banks will never collect bank cards from you, or ask you for your personal identity number.
"If you receive one of these calls end it immediately, wait five minutes to clear the line or phone from a friend's house or mobile
"We are also working with taxi drivers and private hire firms.
"These seem to be being used as unwitting players in these offences, and they could have important information which will help us."
A pensioner who was duped by the gang told the Leicester Mercury earlier this month that he had a narrow escape. At that time police were investigating seven incidents, all in the Loughborough and Market Harborough areas.
The additional 20-plus victims are spread across the city and county.
The 79-year-old told how he was contacted by phone by a conman posing as a police sergeant.
The taxi driver - who was not aware he was involved in a fraud - collected them from the victim's Market Harborough home and was on his way to a house in Leicester to deliver them to the conmen.
However, he was delayed by heavy traffic, giving the victim time to realise his mistake.
The victim and a friend contacted the driver's bosses who were able to contact him to tell him not to make the drop-off.
The man, who asked not to be named, said: "They said they were the police and that one of my cards had been used and charged £700 for a flight to Australia.
"They said the only way they could continue their investigation would be if they had my cards. They asked for my PIN, but I refused to give them it.
"He told me to put my cards in an envelope and eventually a taxi arrived at my house.
"He was taking it to a house on a new estate in Leicester. The police told me they thought the men would be there in a car waiting for the taxi.
"After the taxi had gone I became concerned and rang a friend, who rang the taxi firm and told them to come back to my house and not deliver my cards to these people.
"Fortunately, the driver hadn't reached Leicester yet because he had been held up in traffic.
"I got my cards back and appear to have had a very lucky escape.
"People, especially the elderly, have no idea that this kind of thing goes on."
Anyone with information about this fraud should contact Leicestershire Police on 101, Crimestoppers on 0800 555 111, or Action Fraud on 0300 123 2040.
