This was discovered by Radio Five Live.

Almost a million customers have been sent letters warning them about these fake sales calls.

Customers have complained that salespeople have called at their homes and claimed to be connected with the companies which sold them their telephones.

They try to sell them warranties, some of which have turnrd out to be worthless.

Both companies say they have nothing to do with these sales tactics, and do not give out customers' personal information.

The salespeople can be aggressive and attempt to bully them into buying the cover.

Police and Trading Standards have confirmed they are investigating a number of firms.