The Vigor’s FXS ports are similar but different to a softphone in that they both provide a user interface for making VoIP calls. A softphone just uses a PC’s speakers and microphone and doesn’t require an FXS port, while the Vigor’s FXS ports allow the connection of standard telephone handsets. An ATA device is another common device to allow the use of a standard telephone handset.
Extensions would generally require a VoIP PBX system, but for home or small office use you could wire 3 or 4 handsets together peer to peer style from the FXS port, or just connect a cordless base station that comes with multiple handsets. These just share the one line though and do not allow for multiple simultaneous calls.