An “unlocked” phone is one not tethered to a particular carrier.  That doesn’t mean all unlocked phones work with all carriers.  

And “locked” phones are ones exclusively sold through the carrier.  And some can become unlocked.  

Unfortunately, there’s still exclusivity. Some unlocked phones don’t work with every carrier.

SIM cards are the easiest way to unlock a phone.  When you sign up for a new plan, just show your unlocked phone and they’ll give you a new SIM card.  There may be some small settings changes to make as well.  

But…you have to make sure the phone is compatible with the carrier’s band and cellular standards of choice.  Bands are another word for frequency and standards are the 3G and 4G thing.  

There are two types of standards: GSM and CDMA.   T-Mobile and AT&T use GSM. CDMA is used by Verizon and Sprint.  And neither side are compatible with the other…yet.  

There are a class of unlocked phones that work with both standards.  To check your phone, look up the model number and manufacturer online.  

Try for frequencies and standards.