Abstract: A customized telecommunication service comprising a service related component, referred to as a service shell, and a number of customization parts open to customization by a service provider. The service shell is designed by a system vendor with due consideration taken to the fact that the service must not interfere in a non-desired way with other existing services. The service shell comprises customization points to which the customization parts are added. Each customization point comprises an individual customization interface defining the scope of actions open to customization in the service shell. The service provider selects, among the actions, those which will build the desired function of the customization part.