you have to have an account, with mobile number

Are you sure? I seem to remember Discord requiring an email but not mobile number

I know Telegram absolutely requires a mobile number, though

It will sometimes let you register without a phone number, but other times it will demand one. I imagine it has to do with IP reputation and how many accounts have been created from your network recently.

until the discord server goes down

If the thing goes down there's no guarantee any indexer/scraper is going to index/scrape it