Hi

1.within how many days your IR usage is been charged to you. Check in their portal or CC that within how many days IR usage can be billed to a subscriber.

Idealy if the usage is going absurd beyond your credit limit ideally the services must be restricted.

But again it depends on operator (US) when they have handed overed the usage to Q Tel to bill a subscriber. In case the usage has been received by them they should have barred your no.

Just do your homework on point No.1 then lodge a complaint stating when the cr limit was assigned as 600 then how the usage can go beyond, why the service was not restricted on time.