Q: If someone finds me by username, messages and I reply — will they know my number?
No. Neither party will see another‘s phone number. This is similar to the case when you message a person who you’ve met in a Telegram group.
There is one exception to this: just as in case of all Telegram messages, if you have somebody‘s number saved as a contact and you send them a message, your number also becomes visible to them. Just like with SMS. This happens regardless of how you open the conversation with that person — via the contacts menu, global search by username, telegram.me link, or from a group’s members page.