Theres a weird issue I can't find any solution for: in an organization that is using GSuite admin can't open a mailbox for a fired employee. I mean we can reset password etc but while logging in there's "Verify its you" prompt that will send sms to his private number - that means mailbox can't be opened. We are legally entitled by organization management to open the mailbox, but it seems quite impossible.
It is not a 2SV - we have disabled it for the whole domain in attempts.
Any workarounds?
As an administrator, reset their password, and use one of their backup verification codes.
On top of organization 2SV policy, Google accounts have extra security when faced with suspicious logins. Such as an admin attempting to login with a new device in a different location. Makes sense an unknown device would trigger a "Verify it's you". G Suite administrators can disable this for a short time. (See Mario R.'s answer over at webapps.SE, via this blog.)
Or, skip account takeover. As an administrator, download the user's data via G Suite Data Export.