Get it from the Asset Store:
Overview
This package implements the Telegram Mobile Protocol (MTProto) for Unity. With it you can easily implement user authorization, their communication (eg, in-game chat, …), a variety of social interaction (eg, the publication of screenshots from the game on public channels, …), and much more.
Package contains: client library (53 API layer supported), setup wizard and demo scenes.
Requirements
Unity v5.4.1 or higher.
About Telegram service
Telegram is a cloud-based instant messaging service. Telegram accounts are tied to telephone numbers and are verified by SMS or phone call. Users can add multiple devices to their account and receive messages on each one. Connected devices can be removed individually or all at once.
The default method of authentication that Telegram uses for logins is SMS-based single-factor authentication. All that is needed in order to log in to an account and gain access to that user’s cloud-based messages is a one-time passcode that is sent via SMS to the user’s phone number.
Users can send messages to other users individually or to groups of up to 5,000 members.
Channels can be created for broadcasting messages to an unlimited number of subscribers. Channels can be publicly available with an alias and a permanent URL so anyone can join. Users who join a channel can see the entire message history. Each message has its own view counter, showing how many users have seen this message. Users can join and leave channels at any time. Furthermore, users can mute a channel, meaning that the user will still receive messages, but won’t be notified.
Telegram servers in a number of countries throughout the world to improve the response time of their service.
More: Telegram (software) - Wikipedia

Support Email: contact@neverfall.ru
Beginner Info: Telegram