Check that cable before you assume anything - telephone cable is frequently Cat3, not Cat5 - 10Mbps, not 100Mbps (if you can't find "Cat-anything" on the jacket, it's just about guaranteed to be no better than Cat3). On top of that, household phone wiring topology is room-to-room daisychain, with multiple legs tied back to a common point before going to the demarcation point. Nothing like endpoint-to-endpoint-with-no-branches-or-spurs UTP topology.
You can violate many physical network standards if your runs are short enough. Just don't expect it to be robust.
I used to work in the department that created the software that billed those rebate customers if they did happen to cancel early.
Single tasking: Just Say No.