Do unlocked iPhones use AT&T, tmobile, Verizon and Sprint?

I have a iPhone 4 locked to AT&T but I use straight talk sim. I'm basically just using AT&T service through

So I was wondering what happens if I unlocked my phone. Will I be only using AT&T towers if I keep the AT&T compatible sim card in my phone or will it use all network towers straight talk uses? By the way, I think straight talk should have a sim card that uses all of their towers that they use. My iPhone sometimes don't pick up good service. I feel bad my little cousin's is playing the exact game I'm playing on my phone and gets to the multiplayer part faster than me and be already playing while mine is still loading. But I think the 4G is manly boosting that. Plus he's on Verizon and verizon say they have good 4G. And I use my 3G and draining my battery fast to play. I want to get an iPhone 5 or 5S so I can get 4G. But I'm saving money to do that right now. I'm 17 haha.

When it's unlocked you are free to set it up on any network you choose. You get a sim from them, put it in, and activate. Best luck.

Once the phone is factory unlocked. It pickup whatever sim service tower u put in the phone.

When you unlock it you can use it on any GSM network. You can't use it on CDMA networks like Verizon and Sprint

Unlocked phones can work on any GSM carrier available. Verizon & Sprint are not GSM carriers, CDMA. So its a matter of whatever SIM you insert after it is unlocked.