A method with which cellular network terminals are informed of a possibility to use services of 3G mobile communication networks in a radio access network. A spare bit of the broadcast message SI3 of GSM system is defined as a bit indicating, whether the current GERAN cell supports UMTS service through the Iu interface or not. In a favourable case, another SI message is broadcast including a description of the PBCCH where Iu information for mobile stations is placed. Should the GERAN support also the GPRS service, said another message is SI13, and if the GERAN does not support the GPRS service, said another message is SI13alt. This message can only be employed by UMTS capable mobile stations. By means of the invention the information presumed by the use of the UMTS network can quickly be acquired by a mobile station, which for example arrives in a GERAN cell.