It is common to get a voice message from T-Mobile saying that your credit is low. In most cases this is nothing to worry about as you can still make all the inclusive calls that come with your rate plan.
When we activate your service we top up enough credit to cover your first month of service (if you are staying in the USA for longer than one month we will top up again at the end of your first month). After we top up your credit, T-Mobile will take this as payment for your service leaving your with no remaining credit. This is nothing to worry about as your service is now paid up for the next 30 days, however a zero credit balance is not much so T-Mobile will play a voice message telling you that you are low on credit when you make a call.
In most cases this is nothing more than an annoying message you have to listen to, but you still have all the free calls that come with your rate plan.