That is what the help files say, but when I tried to register a work account yesterday it did the verify you are human, then said there was something suspicious and sent the email verification, then said there was something suspicious and is now requiring a phone verification even though I did not enter a phone number.

At no point was I ever signed in and able to even pick a channel. This all happened while trying to log in for the first time through the browser at work with my work email. I guess that someone else might not hit that phone requirement as I only tried to do the registration once, but it is in no way limited to joining a particular channel.