The id corresponds to the unique identifier obtained during the first call.
The NIP code is first sent by email. In the sandbox environment you won't receive the email, so it does not generate any cost. The NIP used in sandbox is always "1234", so you need to send this value when you are in sandbox to go on with the process.
When this endpoint is called successfully, it will send back a 200 http code, and send the NIP to the phone number registered in the first call.
You then need to call this API a second time with the nip code received on the user's phone. Again, in sandbox, the whatsapp message is not sent for real, so you can just call this endpoint with "1234" as the NIP code, to go on with the process.
If successful again, the process is finished and the user will receive a PDF containing his "reporte de crédito especial".