Sorry for your loss OP.
I think he raised a very valid point, google phone number of Coinbase will come back a fake one. And the fake one will steal your coin.
if this was how OP got socially engineered, that's interesting. it's true that people (including myself) use the top search results to quickly confirm we are entering the correct, legitimate site. i assume the same goes for phone numbers. i google restaurants all the time and order takeout/delivery the same way. if the #2 result is a fake number, we should collectively report the number to google.
the unfortunate thing is that coinbase didn't even have phone support until very recently (the last month or two). so if this is how OP was compromised, this might have been avoided if not for the timing.
i think this guy got his host file hacked and he was trying to log in to a pishing coinbase website is the only thing that makes sense to me
it sounds like he was probably socially engineered into giving up his login/password to the hackers. since he had 2FA on the account, i believe this is the "secret seed" he was referring to. this is a term that coinbase actually uses to refer to the 2FA token used to generate TOTP codes:
https://support.coinbase.com/customer/en/portal/articles/2820377-2-factor-authentication-2fa-faqit sounds like the hackers (on the phone with the OP) got him to verify the hackers' device through email while they were on the phone. it's not 100% clear from the OP, but it seems like he probably handed them his login/password, verified their device for them, and gave them his 2FA token. that's everything they needed to compromise his account.