The VoIP gateways permit you to connect your PBX or your analog telephones on Internet WAN.
If FXS interface are provided, you can connect it analog telephone or external lines (two wires) from your PBX to a LAN or Internet.
If FXO interface are provided, you can connect internal PBX lines (two wires subscriber) to a LAN or Internet, instead of an analog telephone set (two wires).
If PRI T1/E1 or BRI interfaces are provided, you can connect a digital PBX or digital lines from an operator.
If GSM modules are provided, you can connect a GSM network.
Some permit to use DECT/GAP handy telephones as standard SIP IP-phones from Asterisk side.