The super-complaint prompted a 90-day investigation from Ofcom
Regulator Ofcom has suggested that contracts to provide phone services from prisons in England, Wales and Scotland should be renegotiated.
The view comes in response to an official complaint that telephone calls from prison cost seven times normal payphone rates.
The National Consumer Council said that calls were important for prisoners to maintain a link with their families.
Ofcom found that calls made from privately-run prisons were cheaper.
The super-complaint from the NCC said that half the calls made from jail lasted less than three minutes because of the high costs involved.
The NCC and the Prison Reform Trust said that the cost of calls "appear unrelated to the cost of provision", costing the equivalent of 11p a minute.
This prompted the Ofcom investigation into the contracts between BT and the prison services in England and Wales, and Siemens in Scotland.
The high cost is the result of security costs, BT has said
"The evidence we have reviewed indicates that the price of telephone calls made by prisoners appears to be high in particular relative to some measure of costs or compared with the price of prisoner telephone calls in some Serco Home Affairs managed private prisons," the report said.
It said that the average price of a call from regular prisons to a UK telephone number was 60p, but the identical call would cost 48-51p from a private prison.
The regulator has asked the Scottish Prison Service and Ministry of Justice to consider renegotiating the current contracts with the companies to reduce the price of calls.
"If successful [this] would provide a timely and effective remedy to these concerns," it said.
If this did not happen, the regulator said it might reconsider the matter.
The current contracts end in 2011 with the next tendering process starting later this year.
"We are delighted that our super-complaint has finally prompted action to reduce the excessively high cost of phone calls from prisons," said Philip Cullum, acting chief executive of the NCC.
Juliet Lyon, director of the Prison Reform Trust, said: "The high cost of prison phone calls makes family contact much harder, even though family support is a lifeline for some prisoners and those who do keep in touch are less likely to reoffend."
A Ministry of Justice spokesperson said that the Ofcom report recognised the difficulty caused by security issues to provide cheaper phone calls to prisoners.
"We recognise the importance of prisoners keeping in touch with their family, and seeking a reduction in the cost of calls will be a key feature in the forthcoming re-tendering exercise," he added.
"This must, however, be balanced with the need to provide a safe and secure system to prevent misuse of telephones by prisoners."
BT has previously said that pricing was unusual owing to security issues.
A spokesman for the Scottish Prison Service said representatives were considering the report's contents closely.
Relatively few super-complaints have been made since the system of regulation was introduced in recent years. They have included complaints about bank charges, payment protection insurance and doorstep lending.