Based on the error message, it looks like Ringover is using some authorization when making the request to Power Automate. Since your trigger is set to receive requests from "Anyone", no authorization should be provided in this request. Did you try to make this request in Postman or other tool, just to confirm that everything is working fine from Power Automate end?
I'm not familiar with Ringover and I'm not sure about how flexible it is for allowing you to configure the request headers, but if you already tried to make the request via Postman and was successful, I would recommend you to double check the headers (and any other request configurations) to remove any unnecessary authorizations included in the request.
Let me know if it works for you or if you need any additional help!
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