Mobile-phone users are being warned to watch out for rogue salesmen who cold-call customers and trick them into switching to expensive long-term contracts against their will.

Ofcom, the communications regulator, says that complaints about the practice have soared this year and that a voluntary code of practice, introduced in July, has failed to stamp out the problem.

Ed Richards, chief executive of Ofcom said: “In July of this year we put in place a voluntary code of practice, following assurances from industry that they would tackle this problem. I am afraid to say that there is a lack of evidence to date that this will be sufficient to ensure that consumers will be protected. So, this week we will start a formal review into these practices and the effectiveness of the voluntary code.”

The process, known as "slamming", involves switching a consumer to a different network or a longer contract without their consent.

The salesman often promises a free phone upgrade or discounts. The customer thinks this comes with no strings attached but then discovers that he or she has been locked into an expensive new deal.

Mobile-phone companies usually claim that the slammers are nothing to do with them.

However, they commonly do deals with third-party marketing firms to sell their services.

The mobile operators say approved third-party dealers should not try to sell you an expensive or inappropriate new tariff, but instead should only advise you about new offers. Any firms that are deemed to have misled customers will be taken off the approved register.

Anthony Ball of Onecompare.com, a mobile comparison service said: "If the mobile operators were really concerned about this, they would stop paying third-party dealers altogether and encourage their customers to buy direct through one of their approved websites or retail outlets."

HOW TO PROTECT YOURSELF

Never disclose details such as your name, address, or account number to someone who calls you.

Tell them that you are not interested -if you show the slightest interest they will continue to pester you.

If you are slammed you have seven days to terminate the agreement under OFT regulations.

Don't sign anything and keep a copy of all paperwork.

Ask for a contact number. Slammers are very unlikely to give you a way to contact them because they don't want to be caught.

Tell them you will be recording the call -some mobile phones can do this now

If you have a problem, first complain to the firm. Then you can take your complaint to the Office of the Telecommunications Ombudsman (otelo.org.uk) or the Communications & Internet Services Adjudication Scheme (arbitrators.org/cisas).

Anyone can register their mobile number online with the Telephone Preference Service (tpsonline.org.uk). This should stop all UK-based telemarketing calls. But due to a technicality, this will not stop text messages.