Targeting the existing customers of a given company is known as ‘slamming’ and has recently made the transition from urging customers to sign up to longer contracts to urging customers to change provider altogether.

“Landmark has recently entered into a settlement with O2 where it has agreed to put a stop to the practice of mis-selling and has paid £500,000 compensation,” an O2 spokesman told the Guardian.

“We are already involved in pre-litigation correspondence with another company and are lining up several others for actions in the near future,” he added.

Despite the advantages to customers of shopping around before making a commitment to any provider, it is also important to treat calls urging you to switch with caution.

Customers receiving unsolicited phone calls from third party sales companies are advised to avoid disclosing personal details and to remember that under Office of Fair Trading (OFT) regulations, they are able to terminate their agreement within seven days after switching.

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© Adfero Ltd