If you use a internet based phone over your (land-based) broadband service, when you cancel it, your phone will no longer work. If you have a separate POTS (analog) telephone line/service, then you could cancel your land-based broadband w/ no consequences to your phone service.

Also, keep in mind the carriers can detect such unauthorized ethering, and it's quite easy for them to either cancel your service, throttle your speeds or make you pay a penalty fee. YMMV. Also, are you certain your bucket of cellular data can hold up to even moderate internet tethering w/ a PC(?) Food for thought, anyway.