When a user sends a message or when the user adds your WeChat Official Account as "friends", an incoming message from WeChat will pass an unique contact identity for you to reply to. This unique contact identity is not their WeChat ID and it is not possible to request for any personal information such as WeChat ID or phone number of the user.
For that reason, when you try to send a message to a WeChat user using their WeChat ID or phone number, the Conversation API will return error.
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 WeChat through the Sinch Conversation API.