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.
"The chain which can be yanked is not the eternal chain." -- G. Fitch