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An investigation into the soaring costs of certain telephone service numbers has been launched by Ofcom.

The regulator will examine directory enquiries numbers, which begin with 118, after some providers were found to be charging up to £10.50 a call.

It will also look at 070 numbers, which allow users to be contacted on any phone at any location, and can cost up to £3.40 a minute.

The telecoms regulator said prices should be "transparent and fair".

Ofcom, which raised its concerns last week, said there were now more than 400 directory enquiry services offering a variety of options and prices, with call costs ranging from 35p per call to £10.50.

However, there is no stipulated cap on such charges, meaning operators are free to charge up to a maximum of £23.97 for calls of less than a minute.

"Ofcom has been monitoring the costs of the more expensive services, which have risen significantly in recent months as fewer people use these services," .

Citizens Advice has said the current system leaves elderly people particularly vulnerable to high call charges.

Last week, it said it knew of one client who had received a £150 bill for calling a 118 number.

Meanwhile Ofcom said it was aware of one consumer who called directory enquiries in 2009, and ended up with a bill for £350.

When directory enquiries was deregulated in 2003, calls to BT's 192 service cost just 40p.

In a previous inquiry the watchdog decided against imposing a stipulated price cap on charges, but it could now review that decision.

Ofcom also plans to examine 070 numbers, which enable calls to be diverted from one phone number to another, so that the person being called can keep their own number private, and remain contactable wherever they go.

Small businesses and sole traders often use them to make it easier to manage calls.

However, Ofcom said it was concerned they could be confused with mobile phone numbers, which also start '07'.

"Ofcom is concerned about evidence of scams designed to make consumers believe they are calling a mobile number," it said.

"When people call the 070 number back, they are actually dialling a service costing up to £3.40 a minute."

The regulator said it expected to publish detailed proposals following its review later this year.