Help me with vocabulary please?

Dear readers,

I’m asking about a word, but I have difficulty telling you what word I need. Let me try. You see, I have a cell phone, but I can’t make a call or send a sort message. I have to buy a card first, at the back of which there is a number that I need to input into the cell phone. In this case, what do I have to say about what I buy? What am I buying? I buy a voucher card, or I buy …. what?

Another thing to do is to transfer some money to an account via automatic telling machine, then they will send something into my cell phone. When I check it, it says something like you still have 20 dollar and valid until 15 March. So, what do I have in my cell phone that I can use for making phone calls? Is it a deposit worth 20 dollar or what?

Forgive my poor English and expression. And thank you for your help.