Fixed-line phone operators who switch consumers to their service without consent, a practice called "slamming", face hefty fines from Thursday.
Some customers are being switched without their consent
UK telecoms regulator Ofcom has said it aims to stamp out the practice.
Over the past six weeks operators have been drawing up their own code of practice, based on Ofcom rules, to stop the activity.
Firms found in breach of their code could face fines of up to 10% of turnover, Ofcom has said.
The new regime will be in place for two years, after which time it will be reviewed.
Fixed-line phone companies, including the largest operator BT, have welcomed Ofcom's announcement.
Ofcom told BBC News that it was receiving 600 complaints a month from consumers unhappy at being switched without their consent.
However, this may only be the tip of the iceberg as BT has estimated that 15,000 of its customers may be being switched without permission each month.
"Consumers are telling us that the issue of mis-selling is extremely important to them," said Ian Livingston, BT retail chief.
"There are hundreds of customers every day - up to 15,000 customers a month - who are upset about their transfer and our evidence suggests this problem is getting worse, not better."
In the past the National Consumer Council, Which? and Age Concern have all expressed concern at "slamming".
Rogue salesmen need only a consumers name, postcode and phone number to switch them without permission.
They do not have to obtain a signature or any evidence of consent, provided the customer is sent a letter giving them ten days to cancel the switch.