Our 911 systems require a land line. Having used their system twice in the last three years, both times my husband was critially ill and had to have treatment on the way to the ER in the ambulance, no amount of money saved would be worth not having the land line.
We use it a lot for ordinary day to day local calls as well, my cell phone hasn't been used one time in over a month - excerpt for a back up alarm 
We use my husband's cell with unlimited minutes, etc. for long distance calls. It all works out well for us.