This is a pretty loaded question giving the lack of information... What system and what kind of trunks are we talking about here? If you are getting an error message like that, one of two issues is occurring, either your telco does not have that number active, it isn't "your" number, or your system is rejecting the call for some reason such as the number is not in the DID Routing Table. We would need a bit more information to answer that question.