Can you convert a phone jack to become an ethernet jack?

it depends on how the house is wired... if the phone wiring is run from jack to jack in a loop, then no this is not possible - but HPNA might be an option to get a wired network connection to that phone jack... if the wiring is run in a "star topography" (in network terms) or "home run" (in telephone terms) to a central location then yes this is possible... if the phone wiring is anything less than CAT3 it is not recommended. if the phone wiring is anything less than 2 pairs then it is not possible. if the phone wiring is 2 pair CAT3 then the best you can expect is 10mb/s with no PoE if your phone wiring is 3 pair CAT3 with some trickery you can get PoE (4 pair would be ideal) if your phone wiring is CAT5 or CAT6 then all you need to do is change the jacks at either end..