The VoIP-111A is a SIP based advance VoIP adapter that convert 1 analog telephone/FAX into VoIP devices with 1 PSTN backup. Simply plug a normal telephone/Fax into the FXS port, then will turn the telephone/FAX into an IP phone/FAX.The VoIP-111A supports 1 FXS port + 1 FXO port for PSTN backup that can make/receive 1 IP Phone call form different VoIP cost-effective route or PSTN network. The VoIP-111A also comes with IP sharing function so you can share the Internet connection between VoIP and other computers.