Northamptonshire Police are warning people to spread awareness of courier fraud, where criminals pretend to be officials, after reports of the crime increased last month.
In November there were 19 reports of fraudsters impersonating police in scams and police said the majority of these reports came from Kettering and Wellingborough.
This marks an increase in reports as there were only seven reports in October and in September there were only three.
Detective Sergeant Stephen Puddifoot from the economic crime unit said: "The people carrying out this kind of scam are clever and manipulative, leaving victims feeling panicked, vulnerable and frightened that their savings are at risk.
"Those targeted recently range in age from 60 to 95, with some victims losing thousands of pounds as a result - some have been asked to withdraw as much as £15,000 at once."
Northamptonshire Police said: "Courier fraud happens when a victim receives an unsolicited call from someone pretending to be from an official body, claiming there has been an issue with their bank account which requires them to transfer money, or provide financial information, hand over bank cards, cash, or buy gifts.
"The criminal will sometimes suggest the victim rings their bank or the police in order to authenticate the call.
"However, the fraudster does not disconnect the line, so when the victim re-dials they are still speaking to one of the criminals."
In some cases, a courier has even been sent to a victim's home to collect bank cards or documents.
DS Puddifoot said: "It's really important for people to know that police officers will never contact you out of the blue and ask for any financial details, bank cards or cash, nor would they send someone to take you to withdraw cash, buy gift cards or to collect bank cards or information.
"We're encouraging everyone to help us spread the word about this kind of scam so we can prevent any more people in our county falling victim to it."
The police are asking people to tell their relatives and friends that they should never give any information out if they receive a call like this, however plausible the caller may seem.
DS Puddifoot added: "If you're ever in any doubt that you've been contacted by the police, call us on 101 to check the identity of the person who called you - a genuine police officer will never mind you checking they are who they say they are."
Northamptonshire Police said if you receive one of these calls, end it immediately and contact the police and bank on another phone line, using a number you know is genuine like 101 for the police. If you do not have another phone line, wait five minutes after hanging up.
Remember your bank or the police will never ask for your PIN or to collect your bank card. They will never ask you to make high value purchases or withdrawals. They will never ask you for online banking log in details.
Report any suspicious calls or attempted frauds to Northamptonshire Police on 101.