In order to incorporate analog equipment, such as overhead paging, older style conference phones and fax machines into a VoIP or UC solution, an adapter is necessary. Usually taking the form of a small box with inputs for ethernet cable and for the smaller, traditional phone connection. Connection to AC power is almost always required as well. But, with as few as three connections on the Analog Telephony Adapter (ATA), they are simple to hook up.
How is an ATA Used?

ATAs are needed to use an analog device on an UC network. A VoIP phone, for example, has a MAC address that serves as its Internet identity, allowing the VoIP service to “see” it. Analog devices have no Internet visibility, so the ATA serves this purpose. Think of it as the PO Box for the device, which allows the Internet to send and receive information from it.
How Do I Configure an ATA?
The ATA itself, being a very minimalist device, will most often have a portal that is accessed by entering its IP address on a computer connected to the same local network. Since these portals vary by manufacturer and firmware version, a comprehensive guide is impossible. But, luckily, most of the portals have internal help guides to assist you.
Many times, especially on a new deployment, the ATA will be plug and play. Hook it to your network and power, wait until the lights on the face stop blinking (it will almost always update its internal firmware) and connect your phone, fax or other device. Viola! You have a dial tone on the VoIP network.
You will be given advice from the company that furnishes your service if any of the configuration settings within the ATA need to be updated, in the case of number porting, for example. After the initial setup, there is almost no reason you would have to access the IP Portal again.
Fax Machines
A very common application of an ATA is to make a traditional fax machine work over VoIP. Perhaps your company has recently upgraded to a nice networked printer with fax capability. You could keep your analog line and its ties to the local phone company, or you could put it on your VoIP platform with an ATA. Make sure your fax machine supports the T38 protocol, which was designed to enable faxing over VoIP. The ATA itself connects the POTS line from the fax machine into an ethernet cable that goes to your network and eventually the cloud.
Public Address Systems
Retail and office locations with established overhead paging systems will be very reluctant to give them up. Luckily, the usual interconnect is an RJ12 plug, and the ATA can be configured with an extension on your network and then connect to the paging system. It might take a little set-up and perhaps some minor retraining, but it can be added as a favorite on the phones in the office. Once the connection has been made to the paging box (usually by simply dialing the extension you set up) the commands will be the same as the employees already know.
Entry Systems
A lot of door buzzers will have a similar interface as PA Systems (above), and they can likewise get addressed in the same manner. Dial the extension and enter the command. The big difference is that, in the past, there might have been just a button to open the door and in this configuration you may have an extension to dial then a key code. Google can be your friend when the original manual is inevitably lost.
For preserving the functionality of legacy equipment, an Analog Telephony Adapter is an inexpensive and easily configured option. It is not really magical, but it can seem like it sometimes.