I called 2 times today to unlocked my Virgin Mobile iPhone 5s. First time I called, they transfered me over 3 times, anwsering the same questions: phone number, pin, how long I had the phone and if i'm going to a different carrier or international. before they just disconnected. 2nd time they told me they unlocked it, but when I try to sign up for Verizon, it tells me, "The phone associated with the Device ID you entered is not compatible with the Verizon Wireless network." I had a chat with Verizon, and they said it has to do with their towers and CDMA compatibility. If my phone was unlocked, wouldn't it be able to use any service?
The fact that Verizon said the phone is incompatible with its network does not necessarily mean it is locked nor does it necessarily mean it is unlocked either. I know that Verizon 5s models work on Sprint MVNOs (not sure if on Sprint itself) but think Verizon does not accept models earlier than the 6 from Sprint although I may be wrong on that.
The "best" way to test if a phone is unlocked is to try a sim for the carrier you want to use and see if it works.
It is a little complicated because sometimes a major carrier like Verizon will not accept certain models on its own network but those models will work on the company's MVNOs.
You can check the compatibility of different phones and networks here.