Ministers are considering banning GP surgeries using 0870 numbers amid concerns over the cost of patients' calls to their doctor.
About 300 GP surgeries are using 0870 numbers
About 300 of 10,600 UK practices have signed deals with Network Europe Group to install the lines, which are more expensive than traditional calls.
The Department of Health and GP leaders said patients should not be over-charged for calls to their doctor.
But NEG said patients saved money by spending less time on the phone.
Calls to the 0870 numbers are charged at 6.73p per minute - more than double the average rate of calls.
They have proved controversial over recent years as several banks and power companies started introducing them.
Under the deals negotiated by NEG over the last year, a slice of the money goes to the company with the rest being taken by the network provider such as BT or NTL.
GP practices do not pay for the system.
NEG, which spent four years designing the system, would not reveal how much it made from each call but said it was a small proportion.
Customer accounts director Scott Russell said GPs' patients using Surgery Line spend just two minutes on average on each call, compared to 4.5 minutes before.
He claimed the system speeds up the amount of time spent on calls as patients just have one number to call, whether they want to talk to the surgery during the day or a GP at night.
Under standard arrangements, a patient may have to call the GP surgery in order to find out what number to call even at evenings and weekends.
He said: "It is not actually costing patients any more money, we are just taking away some of the profits providers such as BT used to make.
"Nobody complains when they make money, just when we start doing it.
"The truth is patients are receiving a better service."
But a Department of Health spokesman said: "It is a complex issue and we are investigating to see whether 0870 numbers should be banned.
"We do not want to see patients paying over-the-odds when accessing local services."
Ministers are expected to make a decision in the next couple of weeks, although it is not clear whether a ban would affect surgeries that have already signed up, as the contracts last between three and seven years.
Dr Maureen Baker, honorary secretary of the Royal College of GPs, added: "We are not in favour of these phone lines.
"Anything that creates barriers between patients and their GP and reinforces health inequalities is to be discouraged. We would urge other GP surgeries not to take up 0870 numbers."