If you setup username, you are going to get a lot of spam messages. I guess.
Username in telegram is a public identifier that allow anyone to grant the permission to message you.
The other ways to grant this permission are
1. Add contact with phone number (can be disabled in privacy setting)
2. Have a message/forward message somewhere you can see, and the original sender is you. (Identity forward of forwarded message can be disabled in privacy setting)
3. Be in the same group with the target (chat admin can disable member listing)
Notice, this is a one time setup. A account only need to grant this once, and then he can message you at anytime.
Username in telegram is a public identifier that allow anyone to grant the permission to message you.
The other ways to grant this permission are
1. Add contact with phone number (can be disabled in privacy setting)
2. Have a message/forward message somewhere you can see, and the original sender is you. (Identity forward of forwarded message can be disabled in privacy setting)
3. Be in the same group with the target (chat admin can disable member listing)
Notice, this is a one time setup. A account only need to grant this once, and then he can message you at anytime.