Phone carrier lookup service

This is more a question about an investigative technique, but I'm trying my luck…

Does anybody know of a reliable service that would offer the possibility of knowing, based on a landline/cell phone number, who is the carrier of the phone service in Canada/US ?

I have looked at a couple of hits on Google, but nothing reliable came out.

I looked at DocuSearch, which seems to do this kind of lookup, but it only takes orders from inside the US. Since I'm from Canada, DocuSearch is not an option.

Thanks.