Quick question about phone service!?

I don't actually have a plan with AT&T I am just on the next plan paying on my phone as soon as I get it paid off I will be done with them with that being said could I go to wallmart and get one of those $40 cards that are unlimited internet and everything and use it and continue to pay on the phone but not use the AT&T service? Is this possible would it work any advice this phone bill is killing me and it's only me on it $200 for a phone bill with a baby on the way is not working!