A gateway can establish a connection between a circuit-switched network such as an ISDN network and a packet-switched network such as the IP network. The so-called VoIP gateway is a device or a system which bridges the classic analogue or digital telephone and IP telephony. This type of gateway can for example be used to connect a conventional Telephone system to your ISDN interface. The gateway converts the ISDN protocol to Session Initiation Protocol (SIP) and transmits the signals using the IP interface. This function allows for soft migration into the world of IP telephony. The existing telephone system can easily be used with a new IP-based connection and later replaced with an IP-capable system. When utilizing the services of a cloud telephone system, the conventional system can be used until cloud telephony is implemented and then disconnected. To ensure the smoothest possible migration to IP telephony, many internet access routers already feature extensive VoIP gateway functions. Existing telephone systems or telephones can be connected directly to these routers and still be used.