Mxit's API attempts to return the appropraite HTTP status codes for every REST API request.
The Web context is null
The web context is null
Invalid value for messageToSend.To.... The user could not be found on MXit's platform
User has been archived, after 6 months of not logging into the client
OAuth token invalid or expired
OAuth token doesn't allow access to method call
OAuth token presented does not allow access to the method call
Method not allowed
You may be using the incorrect method (ie. GET instead of POST) to access the specified resource.