A device that adapts a subscriber's analog telephones to a digital voice service from a VoIP, cable or DSL provider.  Connecting to the modem, router or directly to the cable coming into the premises, the analog telephone adapter (ATA) delivers dial-tone, manages the call setup and provides the conversion between digital and analog voice.  See 
IP phone and 
VoIP.
VoIP ATA
With this adapter, an analog telephone is connected to either a router (LAN) or the Internet (WAN) for VoIP service.  For telco company (PSTN) backup, it includes an FXO port.  See 
SIP and 
FXS.  
(Image courtesy of Grandstream Networks, Inc.)In this earlier Comcast cable example, the coax line was split between the ATA and cable modem.  The ATA was wired to an analog telephone for voice, and the cable modem connected to the router for data.  Newer Comcast gateways include the cable modem, ATA and wireless router in one device.  See 
cable/DSL gateway.
Connected to the Internet via Ethernet to the local network, this ATA converts IP packets into analog voice (and vice versa) for the cordless base station.  All other phones in this home are IP phones that also connect to the Internet.  The ATA allows older equipment to co-exist with a new VoIP phone system.  See 
VoIP.