Where can I use this iPhone

I was given an iPhone & it needed to be restored before I could turn it on as it was stuck on the apple logo. I was able to do a restore & now it tells me I need a sim card before I'm able to even look at any of the menus on this phone. I called apple to ask about the phone. They told me this was an unlocked iPhone and I was free to choose whatever carrier I wanted. I don't know anybody whose phone uses sim cards anymore these days. There's no marking on the phone as to say if it is verizon, sprint, att ect. I had my mom call verizon as she has a plan w them & was seeing about adding my iPhone. Verizon said they don't activate iPhones unless they came from verizon. Virgin Mobile also told me the same thing. And that both don't do sim cards. I did a lookup of my IMEI number & says the issuer on this phone is CTIA. I have never even HEARD of this company. If this is an unlocked iPhone why am I having such a hard time finding a place to give me a sim card & let me get on a plan so I can use this phone? Anybody know about this CTIA. I did look up their website & tried calling, but of course they are closed outside of Monday-Friday. Anybody know if ATT or T-Mobile would hook this phone up? Anybody have any helpful info about this iPhone mystery for me? Seems odd that it's unlocked but yet nobody will hook it up.Am I only allowed to use it with this CTIA company?

Att and tmobile both use sims.
call att they will hook it up