How long a phone wire can I use for my DSL modem?
We recommend that the phone wire leading from the wall jack to the modem be no longer than ten feet. Any longer, and the wire starts acting like an antenna, picking up random electrical noise, etc. If your phone wall jack is across the room from where your computer is, we suggest using a short phone wire to connect the modem to the jack, and a long ethernet cable to connect the modem to your computer. Ethernet cables are very well shielded, and do not suffer the interference that phone wire does.