I am really puzzled by the idea that we cannot expect a predictable behaviour from our IP PBX regarding answering external sip calls.
I have setup various extensions with their respective inbound routing so they can receive calls from the outside world but I cannot get it always to work.
For example, my extension firstname.lastname@example.org is able to receive calls from ipkall.com but not from viatalk.com which is my trunk provider.
The fact, that it works like a swiss clock with ipkall.com, tells me that the configuration for the extension mr5858-103 on the pbxes.org side is correct.
Can anyone tell me where the problem lies ?. If you speak to viatalk.com they will tell you that their servers are receiving a "addresss does not exist" like message in response.
I understand your predicament, but you need to alter your strategy in order to accept calls via SIP URIs.
Take a look at the solution I mentioned at your other thread you created: "bug outside incoming calls" and next time try to stick to one thread per issue.