A method, arrangement and network entities for controlling supplementary subscriber services in first (1) and second (12) operatively connected, overlayed telecommunications networks. Supplementary subscriber services available to a subscriber (4) in the first telecommunications network (1) may be executed in the second telecommunications network (12). These supplementary subscriber services are controlled by the subscriber (4) from the first telecommunications network (1). The first telecommunications network (1) is provided with a supplementary subscriber services handling indicator (20), indicating in which one of the first (1) and second (12) telecommunications network a supplementary subscriber service of a subscriber (4) is to be executed. Based on this supplementary subscriber services handling indicator (20), while maintaining a user control interface in the first telecommunications network (1), supplementary subscriber services control messages are processed in the respective network (1; 12) in which a particular supplementary service is to be executed.