A Voice-over-IP-(VoIP-) telephony computer system includes a client computer (1) with a web browser (20) installed and which is connectable to a network (4) with access to the public Internet (6), a web server (101) which can be reached over the public Internet (6) from said client computer (1), a gateway computer (102) which can be reached over the public Internet (6) from said client computer (1), a VoIP private branch exchange (PBX) device (9) which can be reached from said gateway computer (102), wherein the VoIP telephone computer system is adapted to run following steps of establishing and maintaining a VoIP telephone connection between the client computer (1) on the one hand and the gateway computer (102) and the VoIP PBX device (9) on the other hand: the client computer (1), using the web browser (20), downloads an applet type application (30), which is adapted to establish a VoIP telephone connection from said web server (101), said downloaded applet type application (30) establishes a tunnel connection (50) to said gateway computer (102) and installs a SIP telephony client (40) on said client computer (1), and all VoIP telephone connection related data between the client computer (1) and the gateway computer (102) and vice versa are transferred using the tunnel connection (50).