Is this not an option?
Telephony Connection Through Computer
https://www.magicjack.com/account/mjLandingpages.do?...
As far as I can see from that:
Option 1: you plug an *analog* phone into one port, and a *wired ethernet* connection to your router in the second port.
Option 2: you plug an *analog* phone into one port, and a *USB* connection to your PC, and run some software on the PC.
Neither option would work with an ethernet-connected SIP phone.