HMRC's numbers are changing, but on this occasion it is simply the telephone variety, as it announces it is to replace all 0845 numbers with the 03 prefix by the summer.

The move is aimed at reducing the cost for mobile phone callers to the tax department, but would not affect revenues as chief executive Lin Homer explained at a grilling by MPs sitting on the Public Accounts Committee.

Homer, and HMRC director general of personal tax, Ruth Owen, were brought before PAC to defend the department's call centre performance following a highly critical report by the National Audit Office (NAO).

PAC chair Margaret Hodge said: 'I think it is unacceptable for a public service to expect the public to use 0845 numbers.'

While calling 0845 on a landline is charged at a standard rate, mobile phone callers are charged a premium rate.

Owen confirmed that the 03 prefix for all HMRC customer enquiry numbers would be established by the summer, before contract talks with HMRC's telephone provider Cable & Wireless are completed.

Owen said: 'We are renegotiating the contract now but we are going to move ahead of that because that renegotiation will take some time.'

One of the findings of the NAO report, published last year, discovered that calls to HMRC cost small businesses and the UK public at large as much as £33m over the course of 2011/12, but while Homer requested not to divulge the profit made by the service provider - citing its significance to ongoing contract negotiations - she acknowledged that it was 'a small profit, considerably less than a million.'

Following the PAC session, an HMRC spokesperson said: 'From April we will start to migrate all our contact centre services from 0845 numbers to those prefixed by 03. This work should be completed by the end of the summer and for the vast majority of our customers means it will cost less to get in touch with us.'