From November 12, the cost will rise from £3 per minute to £5, the company has warned users via a text message. It suggests that, to save money, customers should call its own directory service - the rather unmemorable 118402.
"Until now we have absorbed rising charges from other providers, but we are now obliged to pass them on so that we can continue to offer our customers a choice," says a company spokesperson. "Calls to O2's Directory Enquiry service 118402 are charged at £1 per minute and remain so."
Back in 2002, telecoms regulator Oftel - now Ofcom - opened up directory services to competition, with the claim that this would bring down prices and improve services. However, 12 years on, the market remains dominated by BT and 118 118, and prices are higher than ever - particularly on mobiles.
It's not always easy to compare prices for the various services. All charge a connection fee as well as a time-based price, but while some charge by the second, others do so by the minute, meaning that a 61-second call will be charged as two minutes.
Users that want to compare charges can do so at websites such as Magenta Systems' Telecom Tariffs page, which is updated around once a month. Alternatively - if they have a smartphone - they can just search for a phone number online instead.