Vista Causing IP Phone Disconnect

We use a 3COM IP phone system. Our network cables run first to our phone
and then from the phone to the computer. We have never had unusual problems
with this configuration.

I swapped out a notebook that had XP with one with Vista at one of our
desks. Since we have done this, the phone has been losing a dial tone, and
therefore dropping the network connection for the Vista machine. I have
been unplugging and reconnecting the ethernet cables to the phone to
reestablish the dial tone, and then restarting the Vista machine.

I'm trying to track down a really long ethernet cable so each device can
have their own connection (we only have one jack at the desk).

I can't get my head wrapped around how Vista is causing this problem, but
the timing can't just be a coincidence.

Does anyone have any thoughts?