Abstract: The test facility tests whether the services subscribed to by a user in a subscribed network are also available in a visited network. Because the network functions and services should be tested in the same way as a customer actually uses them, they preferably have to be tested from the subscriber network interfaces. The subscriber network interfaces are especially important at the air interface and at the switched network for conventional analogue and ISDN telephone services (Integrated Services Digital Network). However, for special tests internal interfaces of a GSM network (Global System for Mobile Communication) are also of significance such as the A-Interface at the Mobile Switching Center (MSC) of a GSM network.