A survey from Which? Computing has found that most internet service providers are using premium rate numbers for their technical support.
The consumer group compared the technical support costs of 45 firms.
While a handful used a freephone number, firms including Tesco broadband, Supanet and Roxio use premium rate 090 numbers.
Tesco has said that it is planning to change to a cheaper tariff "as soon as possible".
"The cost of calls to Tesco broadband technical support is something we've been looking at and listening to customer feedback on," said a spokeswoman for the retail giant.
"As a result we'll continue to provide a very high quality of technical expertise to all our broadband customers but calls will now be charged at a local rate.
"We'll be contacting all our customers as soon as possible to let them know about the change in price," she said.
Six firms including BT, O2 and UK Online, offer a freephone 0800 number.
But 32 of those surveyed used higher-cost numbers for their helplines.
"Some companies are using higher-cost numbers at their customers' expense," said Sarah Kidner, editor of Which? Computing.
" If other companies can offer a freephone number, why can't they all?," she said.
Those using premium rate numbers told Which that they did not profit from the calls, but used the revenue to "cover the costs of a technical support service".
The survey also found that nearly a third of broadband users were frustrated by customer support.
Time spent waiting to be connected does seem to have improved. Only 12% were dissatisfied and the survey found that the average waiting time was just 1 minute 33 seconds.
Plusnet customers have the longest average wait - at nearly eight minutes.
The Which? Computing survey looked at 45 technology companies, including ISPs, software and laptop manufacturers.