Every time I try putting in a sim from another carrier it continues to say "sim not supported"

I recently sold me Iphone 6 to someone I know. The phone is from boost and I had already called to get the phone unlocked about two weeks ago, and it isn't attached to any account. For some reason every time I try putting in a sim from another carrier it continues to say "sim not supported" is there anything more I can do?

[Re-Titled by Moderator]