If you have recently topped up with credit to the value of your chosen plan, then your plan is active and T-Mobile have taken the plan value out of your account (and left you with a zero balance) but given you 30 days of service instead. So, you will see that your account balance is zero, but you have minutes and data available to use for 30 days.

You do not need a minimum balance to use the included data and domestic calls that come with your rate plan. However you do need a balance of at least $1 to take advantage of the free international calls that you get with the Unlimited International feature.