Mobile-phone users are being warned not to fall prey to rogue salesmen who cold-call customers and trick them into switching to expensive long-term contracts against their will.

There has been a sharp rise in the number of people falling prey to the tactic, according to One Compare, a comparison firm. Ofcom, the communications regulator, has also received a large number of complaints about the practice over recent months.

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.

Helen Wallace, 28, a researcher from London, recently fell prey to the high-pressure tactics of a salesman claiming to represent Orange.

“He kept ringing me saying he could offer me a better deal and a new phone. Eventually I decided to agree, but it was more to get him to stop ringing me,” said Wallace, who had been rung three times by the salesman.

She was told she would receive a 10 per cent discount on her bills if she agreed to take out a new phone. What she did not realise was that she was being signed on to a new 18-month contract, which she did not want.

The mobile-phone companies usually claim that the slammers are nothing to do with them, even though the agents often say they are acting on a mobile operator’s behalf. However, they may not be as innocent as they claim.

Mobile-phone companies commonly do deals with third-party marketing firms to sell their services. The saleman receives up to £500 commission from the mobile operator for each sale, though the overall profit they make is around £90 because they have to buy the handsets that they send to the customer.

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.

However, there is evidence that mobile firms have turned a blind eye to the sales practices of some rogue traders. They appear happy to pay commission to those who sign on new customers, however they do it, allowing the slamming market to thrive.

Orange admits the majority of its dealers operate under the umbrella of 10 distributors. There are about 2,500 such sub-dealers selling Orange products. Other mobile firms operate in a similar way.

Rob Kenley at Moneysuper-market, a comparison website, said: “Because the agents do not work directly for them, mobile operators can deny liability. But the salesmen are often acting on the mobile firms’ behalf and are paid by them, even if indirectly.”

Orange said it had terminated relationships with 20 dealers in the past because of malpractice.

A spokesman said: “Orange takes all complaints seriously and will investigate any dealer should it appear they are not providing the levels of service Orange expects.”

O2 has recently taken legal action against a Welsh company, Landmark Marketing Services, which trades as Landmark Communications, for ringing its customers in a bid to switch them to rival networks 3 or Orange. It said it was also investigating a further 60 rogue dealers and had started proceedings against another South Wales company.

But Anthony Ball of One Compare said this would not solve the problem.

He 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.”

There are ways to catch the slammers out. If you are phoned out of the blue it is always prudent to ask for a contact number to ring the caller back. Slammers are very unlikely to give you a way to contact them because they don’t want to be caught.

Never disclose your name, address or account number to someone who calls you, regardless of where they say they are from. Without this information they cannot usually amend your contract.

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.

If you have fallen victim to one of these scams, you have seven days to end the agreement under OFT regulations. Make sure you keep a copy of any paperwork sent and don’t sign anything.

Should the network start to pursue you for any bills owed on the new contract, call it and explain what has happened. It cannot force you to pay because it must prove that you signed up to a contract in the first place — which it will be unable to do.

The industry regulator Ofcom, which has no power to control third-party sales companies, is believed to be working with the Office of Fair Trading and Trading Standards in an effort to clamp down on firms that switch customers without their consent.

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).

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. Rogue sellers often refuse. nTell them you will be recording the call — some mobile phones can do this now.