Slamming has been a regular consumer complaint
New rules have come into force aimed at stopping phone companies stealing custom by mis-selling landline deals.
Regulator Ofcom wants to tackle cases of customers being switched from one telephone provider to another without their knowledge.
This so-called "slamming", as well as other phone line mis-selling, leads to 750 complaints a month in the UK.
Firms which flout the rules can be fined up to 10% of their turnover by the regulator.
The regulator has told companies to clean up their act when advertising their offers.
Other demands to phone companies include:
- Keeping better records of sales and marketing
- Providing information to customers about terms and conditions and cancellation rights
- Adhering to new rules on when they can cancel an order by another provider.
Consumers have complained of receiving calls by salesmen pretending to be from another company.
They have also been given incorrect information about the cost of phoneline packages, and have not been told about any charges that apply if they leave the contract early.
Fourteen companies have been investigated by Ofcom and two fined the maximum of 10% of turnover for breaching existing rules.
The landline phone market has been hit by the increasing popularity of mobile phones, and similar rules have already been brought in to tackle rogue operators in the mobile market.