If you are trying to extend the extension, over the net, to another location. You could use an FXO device, like the Linksys SPA3102 and any FXS device (like a SPA1001 or 2102) at the set end. Of course you'd have to set them up to establish an IP connection to each other. Both ends are designed to work with a SIP server but can be made to do what you want with some patience.