Elderly householders are being targeted by fraudsters claiming to be police officers.
The fraudsters cold call their intended victims and tell them their bank is being investigated and that they need to withdraw money.
At least four households in the Newark area have been targeted in the scam since the middle of last week but, as yet, no one has been duped into withdrawing cash.
Attempts to extort money have been reported in Collingham, Coddington and Thorpe.
Sergeant Paul Cash, of Newark Police, explained how the scam worked.
"Cold calls are placed to an elderly victim from someone claiming to be a police officer, sometimes (they say they are) from Greater Manchester Police," he said.
"They tell them their bank is being investigated and that they need to withdraw some money but do not tell them the reason why."
Sergeant Cash said the bogus officer tells the person that they can check their identity and that it is a real investigation by hanging up and immediately dialling 999.
However, although the recipient of the call hangs up to dial 999, the caller doesn't hang up, and, unbeknown to the recipient if they don't cotton on, they believe their 999 call has been answered immediately and receive the 'assurances' they believe they need to believe the earlier conversation to be genuine.
The bogus police officer asks the person to withdraw money from a bank before telling them that a courier will be dispatched to theirs home address to collect the money for safe-keeping while the inquiry is ongoing.
Although the intended victims in the Newark area realised it was a scam and did not withdraw money from their branch - and reported the matter to Nottinghamshire on 101 -several people in Lincolnshire were duped.
In one instance a victim handed over £11,000 to the courier and, another parted with £7,000.
"We call it the 'Courier Fraud,'" Sergeant Cash said.
"They're very persuasive but a police officer will never ask anyone to withdraw money from their bank.
"Anyone receiving a call like this should not do as instructed but hang up, ensure there is a dial tone and then ring 101.
Sergeant Cash said it was unclear as to how the victims were selected, but it was possible telephone lists of potentially vulnerable victims were circulating within the criminal world.
Nottinghamshire Police has received multiple reports of the scam, as has Lincolnshire Police.