A link Id is a bunch of digits attached to an incoming message that is used when replying to the message(for identification). This is used when implementing On Demand messaging. 

When a user sends a message to your short-code, one of the parameters sent to your callback URL is a Link Id which is included in the Response you send to us.
If you use the wrong Link Id the message fails with the errror response "InvalidLinkId".

Here is an article about more reasons that a message might fail.

For further information reach out through support@africastalking.com

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