Q: I bought an unlocked iphone 6, seller verified unlocked status with verizon. Phone activates with at

Connect the phone to a computer with a current version of iTunes (11 or 12), back it up, and click Restore iPhone. This will replace the IOS image and hopefully overwrite the AT&T settings.

You will need to get the original carrier to unlock it.

Where did you find those "details"? The ONLY accurate source for lock status is Apple. If you went to a web site and entered your IMEI you are the victim of a SCAM. No site can actually know that information. So call Apple and ask. I have seen no credible reports that describe phones that are unlocked for US use and locked for international. The opposite (Locked for US, unlocked rest of the world) is common, especially for Sprint phones.