County Durham and Darlington fire service has warned people to be on their guard against bogus callers.
The warning comes after a resident received a call from someone claiming to be from the service, wanting to check the smoke alarm in the property.
After becoming suspicious, the resident, from Bishop Auckland, alerted fire officers.
A spokesman for the force said he was "appalled" and urged people to check visiting fire staff were genuine.
Keith Wanley, community safety manager, said: "I would urge all residents to question individuals who telephone or visit a property and state they are from the fire service.
"Residents should be aware that the fire service are visiting properties to offer home fire safety advice, however all our staff will be wearing uniform and identity badges."