"... if you do not have cell service then it is useless..."
Maybe I misunderstood this. If you meant that trying to call when you are outside the range of cell phone towers, that is true.
But if you meant you had a cell phone but had no purchased service, so you couldn't make a call, that is incorrect.
As long as you have a charged battery in your phone and are within range of towers, you can call 911 without having active service, such as Verizon.
Because of this fact, people who have upgraded and no longer use their perfectly good old cell phones are encouraged to donate them to places that give them to the elderly or abused women who may need to call 911.
Here is one article on it:
http://www.charityguide.org/volunteer/fifteen/cell-phone-recycling.htmSue