Real-time voice or video communication between users on mobile devices or computers
This is only one of possible reasons why your Skype calls to US numbers fail.
Last year, Microsoft has disabled any Caller ID in Skype. This means that all Skype calls are now anonymous/private. If the canadian phone numbers accept anonymous calls, while the US numbers not, then this might be the reason.
Unfortunately, there is no dedicated Skype support anymore, so the only available option is to contact Microsoft support.