The UK's energy suppliers need to make it easier for customers to contact them without facing additional costs.
This is according to research by Which? that found there is a wide variation in the phone charges issued to new customers, existing users and for complaints.
It noted 20 of the 34 energy suppliers, banks and insurers it examined ask consumers to call an expensive 0845 number in order to make a complaint.
Executive director of Which? Richard Lloyd said: "It is unbelievable that companies would add insult to injury by charging their customers a premium to make a complaint."
He added the practice of offering free 0800 numbers to new customers but not existing users - used by Scottish Power - also needs to change.
However, energy suppliers British Gas, Cooperative Energy, EDF, npower and Ovo were all praised by the firm for offering 0800 numbers for all three services studied.
Recently, SSE also pledged to introduce 0800 numbers as one of ten commitments to improve the quality of its customer service.
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