The UK telecoms watchdog has launched a full-blown investigation into the possibility of "deliberate misconduct" by directory enquiries firms.
It is also reviewing evidence of customers being scammed into thinking they are calling regular rate mobile phone numbers.
Earlier this week Ofcom said it was firming up plans to investigate the directory enquiries market. This came after it emerged some firms had hiked rates over recent years.
The lowest minimum charge among the big four providers is £5.50.
Ofcom said it is "concerned about the rising costs of calling certain telephone service numbers". It has been monitoring the costs of the more expensive services, which "have risen significantly in recent months".
The regulator said in a statement:
Our Call Cost Review will consider whether consumer harm is arising out of deliberate misconduct or market failure, and what action may be required. We expect to publish detailed proposals later this year.
070, or "Follow-me" numbers, will also be probed. Follow-me numbers allow users to be contacted on any phone at any location. They are sold on a one-off basis, for example when someone is buying or selling a car, and does not wish to advertise their private mobile or fixed line number on a website or magazine.
The problem is that 070 numbers could be confused with mobile phone numbers. While calls to mobiles are often included in packages, those to 070 are considerably higher, costing up to £3.40 a minute.
The watchdog said: "Ofcom is concerned about evidence of scams designed to make consumers believe they are calling a mobile number."
The directory enquiries sector is dominated by four players. Alongside BT and 118 118 are Yell (118 247) and Maureen (118 212), which have minimum charges of £5.50 and £6.98 respectively.
Competition in the sector was introduced in 2003, when it cost 40p to dial BT's 192 directory enquiries service.