If you are on a spending limit account, you must pay that bill down below that limit before the due date, or you line will be hotlined.
Example: If your bill is $200, and your ASL is $150 you must pay that $51 before the due date, or your service will be hotlined.
And by "hotlined", you mean service will be cut off, right Watts? It's not that unusual, azaragoza, so don't let it bug you. Just knock the amount down with a payment like Watts says.
Hotlining an ASL account entails the device not being able to make calls or send texts. All recieving functions remain active, until a certain point where the account is considered delinquent.