IPhone 5 bought from ST

A couple of years ago, I bought a new iPhone 5 from Straight Talk. Four days ago, the battery swelled right up, and Apple said to turn it in to Apple for a new phone. I did so, and now I have a brand-new iPhone 5. The original iPhone 5 did not have a Sim card, and neither does the new one. But Straight Talk will not activate the new one unless I get a Sim card for it, which I just don't understand. However I called tech-support and they said that they would mail me a Sim card. I have no idea why my phone now requires a Sim card, when it never did before. And the service people that I talk to must be
in the Philippines, because they're very difficult to understand.
