It is likely the + on inbound is breaking something. (+442033720303)
You can modify [trunkinbound] in extensions.conf which has a section to remove the + (we have a couple posts dealing with this as well, just stripping the + so it will match a DID).
But that may never get a chance to fire since the system says it will not authenticate. Any time authentication becomes an issue, you should modify your carrier SIP context to include the IP address and NOT include a user/pass or to have "insecure=invite" as mentioned above.
Registration is at the discretion of the carrier. Some allow you to "point" the DID to your server's IP address, some do not. If they require registration that does not mean you need to put a user/pass in the SIP context for that carrier. The register => entry is completely separate from the [context] for that sip connection and can even have different authentication information. I have a carrier who requires this as we register to one server, get calls from another server (several, actually) and send calls through yet another server (or two, depending on the coast of OUR server).
Learning how to make it work for each individual carrier can be painstaking, but it often helps if you just call the carrier and ... ask.