Is the iPhone 7 version A1778 still able to be activated?

I’ve been trying to sell my Aunt’s old iPhone 7, but it’s the version that only works on GSM carriers and not CMDA carriers. I’ve been trying to verify if it’s unlocked, so that the new owner can connect to their network if it’s a different GSM than AT&T (the network the phone was originally connected to). But now upon trying to turn on and activate it gives me an error message. Is there any way I can verify this phone is unlocked without having to call AT&T? My carrier is Verizon, that’s why I couldn’t use this phone in the first place. I’ve tried checking the IMEI number online and it just says “unknown”. The phone has no issues and I’ve been able to get through the setting up process after resetting multiple times to go back and check certain settings, but this time I couldn’t even set it up. Please help.