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.

You get this message because you SIM card comes with only $3.34 credit as standard and when you top up with $60 credit, this $60 is used to pay for your rate plan (if you're on the Unlimited Talk, Text & Web plan with Unlimited International feature) leaving you with the $3.34 that you started with. $3 isn't much so T-Mobile plays 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.

Assuming you're on the Unlimited Talk, Text & Web plan with Unlimited International feature, you will only use your remaining credit if you send a picture message outside the USA, call a mobile phone outside North America or roam onto a mobile network outside the USA. If you're not going to be doing any of these things then you don't need to add more credit as it doesn't matter if you have a low balance (although you do need to have at least $1 credit in your account to take advantage of the free international calls that come with your plan).