Slamming is the mis-selling of legitimate products. Customers are simply switched from one company to another without their consent - for example switched from one electricity supplier to another. It occurs because staff are highly incentivised to meet their targets.
Callers may pretend to be from a particular company when they aren't; they may ask you if they can send you information, and if you agree will mark you down as a customer who has agreed to switch providers. Callers may make exaggerated claims about the money you could save if you switched or may offer you an incentive to switch - for example free energy for 6 months - but this never materialises.
trueCall's Call Recorder memory card allows you to record you telephone calls and prove exactly what was said. trueCall customers have won back thousands of pounds of compensation from companies who have slammed and mis-sold in this way.