First of all, to call the /call-out endpoint, your app must be authenticated by the owner of the device which will initiate the call. In other words, only the owner of a device can call the /call-out endpoint to dial from that device.

Looking at your code, I don't know which device id you are using to make the call. Because you are reading the account device ids. You should either make sure the device is owned by the authenticated user. Or better, you should use the get extension device list API to read the device id of the authenticated user and choose the device id which is online to make a call-out.

Let me know if that solves your problem.

Spot on - I was using the admin account to authenticate and initiate the call, which is why I was getting the 404. I can't help but feel a 403 or 401 would have been a more descriptive response for this though as the device does exist, the account just didn't have permission to use it?

Either way, I appreciate the help!