Changing iPhone from US Cellular to Tmoble?

I'm not finding any answers to this basic question anywhere. But I have the iPhone 5s from US cellular that I absolutely love. I'm currently with tmobile on the prepaid plan with a not so great phone. So I wanted to unlock the iPhone to make it a Tmoble phone.
It doesn't have a sim card, I don't think.

I called a store that unlocks phones and he said that you can't unlock sprint or is cellular phones but almost everybody else is telling me differently.

If it dosn't have a sim card then it is a cdma phone which will not work on the t-mobile network.

You can't unlock cdma phones. So Ignore what "everyone else" is telling you.

While you can unlock CDMA phones, they can only work on another CDMA network and only then if the carrier you plan to use it on, is willing to activate some other carrier's CDMA phone, which most are not willing to do.

In the US, most of the CDMA sub carriers are part of either Sprint or Verizon to start with. So those carrier may allow it, but most would rather you use one of their phones.

As Jon Mac said, it'.s a no go going CDMA to GSM, unless the phone in question happens to be a WORLD Phone (international), having a SIM slot for using on Carriers in other countries. Most of the "WORLD" phones were the HIGH end phones, sold by Sprint/Nextel to their business clients.