It is not possible for business to send a message to a Telegram user using their Telegram ID or phone number.
When the user adds your Telegram Bot, an incoming message from Telegram will pass an unique contact identity for you to reply to. This unique contact identity is not their Telegram ID and it is not possible to request for any personal information such as Telegram ID or phone number of the user.
If you need to verify and merge the user with your CRM, then you can authenticate by asking them to scan a QR code that include their account information. After the authentication, you can update Conversation API via Contact Management API. The authentication is done solely on your system's backend and is not part of Conversation API.
The Sinch Developer Documentation site contains comprehensive documentation and tutorials, which show you how we support Telegram through the Sinch Conversation API.
