According to the SIP.US site, the outbound number format looks to be correct.
Usually a "not found" suggests that the number format is incorrect, but it may also mean an issue with the caller ID you are sending
Does the trunk actually show as registered?
Assuming this is a new service, and this is the first time you've used it, there may be "something" that has not been enabled at your providers end. I would contact them to see if they are able to help you troubleshoot, before spending any more time chasing your tail.