Learn to read something like GSMarena. It will give you all the data about the phone which you will not get on these sites.
(I've never seen band 12 TMOUS covered or any discussion of it.)
Then you need to go read the carrier's advertising, gotchas, and check some of their columns.
Sprint does or did do things differently. I think they still don't allow unlocked phones, and they were using a different type of calling from when they bought Nextel. You need to read before you sign up. Be a pest if you need to.
Use Sprint's or a general Telco type forum to complain. You will get better answers.