Patients have paid more than £1 million in call charges to NHS Direct, research suggests.
But both the health service and BT deny having seen any of the money.
Most 084 numbers divide the call revenue between phone providers and the body receiving the call.
David Hickson, who campaigns against such numbers, has calculated that NHS Direct should have received nearly £1.1 million in revenue from calls to its 0845 number.
But NHS Direct says that, in line with government guidance, it does not receive this money 'either directly or indirectly'.
A spokeswoman said it was paying 'the market rate to BT for the usage of our 0845 service'.
BT said it could not discuss a commercially confidential contract. But sources close to the company deny it is making excessive profit from NHS Direct patients. Instead they suggested the contract was structured to take account of this revenue.
If true, this would mean NHS Direct was indirectly benefiting from these charges.
'That million pounds is going somewhere,' said Mr Hickson.
Meanwhile, the DoH has delayed plans to move NHS Direct to a cheaper number - even as it pushes practices to abandon their 084 numbers.
Last February health minister Ivan Lewis gave a 'cast-iron guarantee' that the cost of calls to NHS Direct would be slashed. He said it would not be allowed to continue charging high rates.
NHS Direct later reserved a cheaper 0345 number. But a DoH spokesman says the move is on hold while it considers creating a national three-digit number for urgent care.
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