A public spending watchdog is investigating claims that the use of premium-rate phone lines by government departments amounts to a “telephone tax”.
The National Audit Office expects to deliver its verdict in July, head Amyas Morse told campaigning MP John Healey.
The Labour former minister led calls for a probe after exposing the extent of the use of such numbers.
His research found the high-cost lines account for 61% of all telephone lines at 16 government departments and agencies.
Calls to 084- numbers cost up to 41p per minute and the Department for Transport earned £2.5 million from calls in two years.
The Home Office topped the list, with 96% of its lines being high cost: 22 out of 23 lines.
The Department for Work and Pensions was singled out for criticism given the number of low-income users likely to use its phone lines.
Services charging 0845 rates include sections dealing with disability living allowance, carers allowance, job centres and the social fund (can provide help for people on low incomes who are facing emergency expenses), as well as at the Pension Service, Insolvency Service, Rural Payments Agency and the Skills Funding Agency (houses the National Apprenticeship Service and is tasked with increasing apprenticeships).
Work and Pensions Secretary Iain Duncan Smith has since confirmed that new benefit helplines. for personal independence payment and universal credit, will also use 0845.
The HMRC also pocketed £4.7 million worth of free services offered by Cable & Wireless in exchange for 400 million minutes of calls to 0845 numbers.
Mr Morse, the Comptroller and Auditor General, told the MP it would consider the results of his research as part of its investigation.
"The study will report on the reasons why different numbers are used, the benefits and burdens, and plans for the future," he said.
"We will assess why higher-rate numbers are preferred to geographic-rate or freephone numbers, and the extent to which departments achieve their share of financial and other benefits from higher-rate lines."
Mr Healey said: "I am pleased the NAO (National Audit Office) is carrying out a thorough investigation and look forward to their findings.
"So far the Government have shown no understanding of how unfair this practice is or the problems they are creating for people on low incomes.
"People have no choice about making these calls to get information and help, change details, make claims, but are charged rip-off rates to do so and end up with big phone bills they can't afford.
"We already make millions of calls to government departments and things are set to get worse. Upheaval and turmoil in the benefits system will bring an explosion of inquiries and problems.
"The Government could make things easier for people but, instead, they've set up their new helplines as costly 0845 numbers.
"Members of the public are subsidising the Government's phone bills to the tune of millions of pounds and phone companies are making a fortune.
"Living costs are high enough already without the Government hiking up phone bills through this telephone tax."