After a chat with admin, I demonstrated i'm indeed who I said thanks to Name and Surname, and a selfie.
Hope will be useful for someone else!

Hello,
I'm in the channel YDX in slack.com, but unfortunately my phone died, and obviously I didn't save the backup codes (ok, i'm stupid, I know!).
A friend of mine is in there to, and he asked to the admin (dunno if I'm worng "Come-from-Beyond" ??) if it was possible to deactivate the 2fa for my account.
He said that for a security reason (that I can almost understand), he can't do it unless we give him better arguments.
I still have the access to my mail, and that is the only proof I can give about my identity, since slack doesn't ask for phone numbers, ID cards, or anything. My mail and slack account passwords are differents.
Can you help me understanding how can I prove them I'm the owner of that account?

Thanks.

Riccardo