Telegram uses an SMS-enabled phone number for account creation and as your default user identifier. You have the option of creating a username which others can then use to contact you with (without revealing your phone number), but this is an additional step that most users are not going to take the time to do. It also doesn't seem possible to add usernames to your contact list–which seriously limits the utility of usernames.

To start a conversation, you select an entry from your contact list or search for a user by their username (if she has chosen to use one). Searching by phone number doesn't seem to work. Usernames are public, and there doesn't appear to be a way to block incoming messages from specific users. There's no reason to think that you can't send messages to random numbers.

The contact list in the Android app is sourced from your phone's contact list. Telegram-enabled contacts appear at the top of the contact list and have a graphic next to the name. You can invite other users to sign up for Telegram, which sends them an SMS from you directing them to get the app. (The version of the app I used failed to send SMS to invitees unless Google Hangouts is the default SMS app.)

There doesn't seem to be a way to add usernames to the contact list.

TODO

TODO

TODO