What is an ATA (Analog Telephone Adapter?

An analog telephone adapter (ATA) or foreign exchange station (FXS) gateway, is a device for connecting traditional analog telephones, fax machines, and similar customer-premises devices to a digital telephone system or a VoIP telephone network.

An ATA is often built into a small enclosure with an internal or external power adapter, an Ethernet port, one or more foreign exchange station (FXS) telephone ports. 

An FXS interface is any interface that functions like a standard telephone line jack on the wall.

The digital interface of the ATA typically consists of an Ethernet port to connect to an Internet Protocol (IP) network.

Using such an ATA, it is possible to connect a conventional telephone to a remote VoIP server. The ATA communicates with the server using a protocol such as SIP or others and encodes and decodes the voice signal using a voice codec such as G.711, G.729 or others.

Since the ATA communicates directly with the VoIP server, it does not require a personal computer or any software such as a softphone. It uses approximately 3 to 5 watts of electricity, depending on the model and brand.

Sources:

Analog telephone adapter - Wikipedia
https://en.wikipedia.org/wiki/Analog_telephone_adapter