A VoIP network is an IP network. However, it has some components that support signaling and provide for the transition to fixed networks, mobile networks and terminals.
For example, in a VoIP network, the gatekeeper controls signaling, and H.323 gateways handle the transition to fixed networks, local networks, and terminals. In corporate networks, IP telephone gateways take care of routing telephone calls through PBXs. These gateways must also support signaling protocols such as the DSS1 protocol and the QSIG protocol, as well as proprietary protocols. To connect the PBXs, the VoIP gateways are equipped with the S0 interface and the S2M interface.
Instead of the VoIP gateways, which perform a pure translation function, Session Border Controllers( SBC) are also used, which offer a greater variety of functions and data security.