They're also making moves to make the phone number requirement unnecessary. What privacy features does Telegram have? It sounds like they don't even have encryption on by default and people have also dismissed their security? Why would anyone use them?
Only the expert's opinions are of any value IMO, and I've never seen anyone showing an attack on Telegram's encryption. Telegram themselves seem to claim that it's never broken. I often see vague criticism over the fact that they use their own protocol, but never anything more detailed than that.
https://core.telegram.org/techfaq#q-i-39m-a-security-expert-...