Board Thread:General Discussion/@comment-8-20190712165708/@comment-5811199-20190716075744

I also didn't have to provide a phone number when creating my account. If you're using a VPN or public network, they would have reason to flag your account as potentially being used for botting or other nefarious reasons.

That thread Anonminati linked has a solution which is to contact Discord support if phone verification is not an option for you.