I have a similar problem to this one, but if I try to subscribe the phone number using phone id I get the following error:

Unsupported post request. Object with ID 'ID_HERE' does not exist, cannot be loaded due to missing permissions, or does not support this operation.

So, basically, I've successfully subscribed the app first and I did not receive any webhooks. Then I found your solution and tried to subscribe the phone number, but it seems that it is not working.

When I debug the token in the Access Token Debugger, it is clear that it has both WhatsApp permissions, but this is not the only endpoint for which I get the 'missing permissions' error, which is weird to me. Do you have an idea what could the issue be here? Thanks