This device has two FXS ports (emulate a phone line and connect to an analog phone). If unlocked or you can figure out how to unlock it, you could use it to connect one or two analog phones as extensions on FreePBX.
It cannot be used to connect to a copper PSTN line or to a device such as a cable MTA or fiber ONT that emulates an analog line. For that, you would need a gateway with one or more FXO ports (emulate a phone and connect to a phone line). Such devices include Grandstream HT503 and HT813, Cisco/Linksys SPA3102 and SPA3000, Obihai OBi212 and OBi110.
Instead of a gateway, if your desktop will accept it, you can use an analog interface with one or more FXO ports.