Well, I already paid the first invoice when I bought the Sim itself, and it's being shipped to me, and I'll get it activated once it arrives, but you can also use something like T-Mobile prepaid as they have gs for it on coinsbee.com. That's what I've been using up until now and the reason I'm making the switch is because I want to be able to pay my phone bill directly with Monero instead of having to buy a gift card and then pay it with the gift card. So it just takes one step out of the process.