4 pane comic of dolan on the left and spooderman on the right
pane 1 (dolan): cum join opensurce cummunity!
pane 2 (spooderman): shure! how joyn?
pane 3 (dolan): Here discord! (with discord logo)
pane 4 (spooderman with tears in eyes): y u do dis?
Sometimes it depends on discord itself finding you suspicious, for some definition of suspicious. perhaps a user agent whitelist? lack of Google cookie?
Its a moderation tool. Server admins can choose to only allow users who are verified by a phone number.
I've had it happen on servers where that moderation option is not enabled. My worst experience was trying to join a friend group's discord via an invite link shared with me. I was prompted to create an account with email, and I did. I was then shown a read-only view of the server: I could see all messages and other folks could see I joined and 👋 to me. I could not send messages myself, however, without verifying with a phone number. Further, I couldn't use a Google voice number (my primary number) to verify, nor my "real" number which was associated to another account.