Reports of cold callers phoning Surrey residents to sell them home security systems are being investigated.
Surrey Police and trading standards officers said there had been several reports of incidents in recent weeks.
The residents have been contacted by someone who asks them to take part in a survey, tries to sell them products, and then offers a free burglar alarm.
But the callers do not mention the £300 installation charge and £5,000 maintenance costs, police said.
James Robinson, from Surrey Police, urged people not to enter into any agreement without careful consideration.
He also said anyone who had been contacted by a company claiming to be police approved or offering a guaranteed police response should treat the call with caution.
People could obtain independent home security advice from the police force by calling their local crime reduction adviser, he added.
Steve Playle, trading standards officer at Surrey County Council, said: "Our advice would be not to deal with cold callers who either telephone or come to your doorstep.
"If you are looking to install home security, research the providers available independently and ask them for some quotes."
He also said people installing security systems should be clear about installation costs and ongoing charges which may not be obvious.