If it was a mistake where they were only charging you a few bucks less then you should have been, I would let it slide,
.
But since this is an entire plan, I would call them. They would either backcharge you cancel your account. Then you may have to deal with the whole bad credit issue, which I think would apply. This person still owes Sprint X amount of dollars and didn't pay so we shut the number down. Would be hard to get an account with another carrier.
You can still get sero though.