I'm trying to follow this guide to start business initiated conversation in WhatsApp. I successfully created the user in the first step, but I cannot link it to the WhatsApp channel.
I'm calling API POST request:
https://api.smooch.io/v1.1/apps/{myAppID}/appusers/{myUserID}/channels
With body:
{
    "type":"whatsapp",
    "confirmation":{"type":"immediate"},
    "phoneNumber":"phoneNumberInInternationalFormat"
}
But result is:
{
    "error": {
        "code": "bad_request",
        "description": "Linking not supported for whatsapp sandbox integrations."
    }
}
So my question - is there any way to send a message using Sandbox number, or I need to register a number by requesting access process?