Telegram uses an SMS-enabled phone number your account identifier. This isn't the most comfortable idea but is increasingly common with mobile-first apps and services.

However, with Telegram things seems to go beyond just using your phone number as an identifier. It seems anyone who has your phone number can initiate a session with you. While this is how conventional telephony and SMS work, I'm not sure it's the best model for modern, secure, and private communication.

To start a conversation, a user must be in your contact list. By default, you have no contacts. The app has a feature that lets you invite users to Telegram in the form of a list of contacts from your contact list. When you ask the app to send an invite, it will send them an SMS directing them to get the app.

• The version of the app I fails to send SMS to invitees unless Google Hangouts is your default SMS app.
• It's unclear at present if the app shows you whether a user in your contact list is Telegram-enabled or whether you need to discover this by attempting a session.