Here are more details about that[1]. I'm still surprised how they could have gained access to his Youtube and Twitter just by using his phone number.
[1] https://www.reddit.com/r/boogie2988/comments/4psg4x/i_was_ha...
[1] https://www.reddit.com/r/boogie2988/comments/4psg4x/i_was_ha...
Being able to verify a code sent to the mobile phone registered with the account is used as proof of identity for account recovery by basically everything online except banking.