Problems to solve incognito icon Sim another company my Verizon HTC ONE M8.?

Good friends, I have a problem reading Sim.
to put the sim in my HTC ONE M8 a message that is not Verizon, then an icon appears above SIM me htc6525lvw Verizon appears to me they are a sign of incognita. Whether you upgrade from 4.4.2 to 5.0.1 and the same icon just keeps popping up, so I can not use my 4g LTE network telephone company, I have another cell of my brother same brand of AT & T 4G rises smoothly. I hope you can comment me if you have any solution.

How do I unlock to use with any company?

Hey guys I just got my AM7 from a carrier that I don't use , I want to unlock it...
My question is, those sites that provide codes to unlock the phone are trustworthy?
Or by just flashing a stock firmware it'll be available for any carrier?

Regards.