Mobile phone companies may have to alert consumers via a special dial alert that they are about to call another network.
Calls to other networks have traditionally been more expensive than calling someone on your own network. Under a draft direction from ComReg, consumers must be informed when they make calls to other networks, known as "off-net calls".
ComReg is asking the phone companies for their views on the draft direction with a view to implementing the change some time this year. ComReg said it was vital that consumers be provided with "clear tariff information in a meaningful format".
The chairman of ComReg, Mr John Doherty, said it was important consumers were protected in an "increasingly competitive environment".
It said this was necessary because the prefix before the number dialled did not necessarily indicate what network was involved.
The change will involve the three mobile phone operators - O2, Vodafone and Meteor - introducing what ComReg described as a "tone alert" which will kick in as soon as someone starts keying in a number belonging to another network.
ComReg said this would give consumers a chance to abandon a certain call without incurring any charge.
It added: "This aural signal will be generated and inserted by the originating mobile network and receipt by the customer will not be dependent on the functionality of the originating customer's handset".
ComReg said not all people in the industry favoured the tone alert idea, but it added: "There is a clear need for a measure which will eliminate the risk that consumers may be faced with higher call charges than they anticipate when they initiate a mobile call."
The issue of "mobile number portability" has become controversial in the mobile telephony market. Customers who change from one network to another can bring their old number with them to the new network. Since July 48,000 customers have changed network. ComReg said it was vital these people were aware that calling their old network could be costing them more money than they realised.
