I am https://t.me/AmindaEU at Telegram, but as per their username policy[1], I am unable to reserve the name there due to not being a user of Facebook, Twitter or Instagram and I have no desire to become one.
This means that theoretically anyone could takeover any Telegram username I register and thus my account there should be confirmed outside of Telegram to ensure I am being chatted with, not someone who registered my username at two of three of those services and thus got the username transferred to them.