I have configured my application to receive callbacks on sandbox request api status changes, but I have not been receiving any call to my webhook url. I have been able to test my web hook url with a post and it works so I dont think its the url that is the cause of the issue.
Note I am able to change the status through a Put request to the sandbox and afterwards on polling the request endpoint I see the status changes I made. Yet no call to my webhook url set in the config.
Note the url I used as the web hook in my app settings page is a subdomain with this format: https://subdomain.domain.com/xxxx/xxxx/xxxx/
I found out you don't get the responses back unless you have a CA signed certificate installed on your server. Hope this helps!