I do not wish to contradict the excellent advice that you have received here. My situation was a little different than yours and so I thought I would throw this in for what it is; an extreme exception. I could not isolate the phone line completely because I have U-verse ADSL coming in over the phone line.
In my place, there is an old 4 wire system. I have no hardwired phone service, but the DSL service does come in on that line so I could not completely disconnect it as recommended. It was necessary for me to physically disconnect the incoming 4 wire cable from the house wiring and connect it to a separate phone jack. I installed a two jack plate for this purpose. The incoming DSL service feeds the first jack and the second jack on the 2 jack plate is connected to the standard house wiring.
The first jack feed the DSL service via a cable plugged into the gateway/modem/router. My OBi100 is connected to the router and the Phone jack from the OBi is connected to the house wiring. My handsets, FAX, desktop computers and wireless phones connect to the standard phone jacks throughout the house. Not an ideal solution, but workable. Since the incoming cable was already split at the primary jack, reversal is easy.
There is no crosstalk or interference issues of which I am aware. But as I posted elsewhere, I was having some quality issues on outgoing FAXes, so I added a inline DSL filter between the FAX and the OBi and that seems to have solved the problem.
