ATA Adaptors are small devices that look like a small switch/router. They contain a connector for your normal landline PSTN telephone to plug into. You can also use a Dect phone unit instead of a normal phone. There is also an RJ45 connect to connect the ATA adaptor to a broadband ADSL router and some have an extra PSTN phone input for your normal home landline to come to the same phone.
Thus an ATA adaptor converts your normal phone into an IP phone type of device. You do not need to have your PC turned on, to make calls from your phone as it is now connected directly to the Internet. Normally it is best to buy an IP phone as there are more incompatibility issues with ATA adaptors. Not all normal phones work well with these and there are also some incompatibility issues with certain ADSL routers.
ATA Adaptors are configured just like IP phones. You can access the web menu by typing the IP address of the ATA adaptor into a web browser. You can change the settings to use Tpad (sip.tpad.com) and add your Tpad number and password in the relevant fields and save the configuration. Here are some common ATA adaptors and pdf versions of their setup instructions.
Tpad is compatible with any ATA that supports SIP