Imagine opening your post to discover you've switched telephone provider without knowing anything about it. That's just what has happened to thousands of customers who've been victims of "slamming".
The telecoms regulator Ofcom describes this practice as "an extreme case of mis-selling when customers are switched from one company to another without their knowledge or consent". The rogue companies don't need your signature or bank account details; your postcode and home phone number will be enough to initiate a switch.
Ofcom says it currently receives 600 complaints a month about slamming, but while the regulator can fine firms that are found to have broken the rules, what can consumers do to avoid falling prey?
Warning signs include automated calls, a "welcome" letter in the post from a "new" provider, or a "goodbye" letter from your existing supplier. "Never ignore these," says Andrew Bradley at the Office of the Telecommunications Ombudsman (Otelo). "Contact your current supplier immediately as it can stop the switch."
Switching takes 10 days; miss this deadline and you will have been transferred to a new supplier. However, the damage can be undone.
"Take up the issue with the new provider," says Eddie Murphy, a telecoms expert at Priory Consulting Services. "Often they are totally reputable – it's the third-party firm they employed to cold-call consumers that has acted badly."
Mr Murphy recommends that people who believe they have been transferred incorrectly should write to a senior figure such as the chief executive or the customer services director of their new provider and ask that the switch be undone immediately. Keep copies of correspondence and send letters by recorded delivery.
But what happens if you incur charges for using the new service while you're in the process of trying to switch back?
"If calls are made, they should be paid for," says Mr Bradley. However, when the transfer is reversed, there should be no reconnection charge. Les King, spokesman for BT, says: "Customers who've been slammed away from us won't pay the £125 reconnection fee. And if we inappropriately raise a charge, it will be refunded."
Providers that fail to correct slamming cases speedily risk incurring the wrath of two key industry bodies. One is Otelo, the other is the Communications & Internet Services Adjudication Scheme (Cisas). Both offer dispute resolution between consumers and providers, and both will normally take up a complaint after three months. They can order firms to put things right and pay compensation to wronged consumers.
Gregory Hunt at Cisas has some simple advice: "If you haven't asked a supplier to contact you, hang up.
"This is not a scam aimed at the elderly or vulnerable; it can happen to any of us."
Mr Hunt says going ex-directory and registering your number with the Telephone Preference Service (0845 070 0707) can minimise cold calling.
'BT said I had left. I'd never heard of this other supplier'
Gordon Gadsby, 71, from Hampshire, had been a BT customer for more than 30 years before being targeted by slammers.
"I had a strange automated call one evening with a message claiming it had information for BT customers," he recalls.
Assuming the call was genuinely from BT, he stayed on the line and was told he could qualify for a discount. "I thought anything to keep my bills down sounded great so kept listening, but then the message asked me to press a series of buttons on my phone if I wanted more details, and at that point I got suspicious and hung up."
Later that week, Mr Gadsby received a letter from BT – "which said it was sorry I was leaving. I got straight on the phone to BT and was told that another company had told them I was switching. I'd never even heard of this other company."
For four days, he tried unsuccessfully to contact the new provider. Finally, he rang BT again, which cancelled the switch.
"What's frightening is that I'm ex-directory and haven't given anyone my personal details, so how could they do this?" he asks.
Mr Gadsby adds: "I had this with the electricity a couple of years ago, so I'm just wondering what they can do next without asking me."