Firstly, if it is showing up as "Unknown" on the Dial Plan, regardless of other issues, you need to fix that. In general, you wouldn't allow folks to dial numbers that IC can't figure out what they are!
At what point do you get "Invalid Phone Number"? When you add the entry, or when you try to use it?
If the former, then my guess (although I am not certain and am not in a position to test it right now) is that the process does run the number through the Dial Plan to categorize it and, because it's "Unknown" is refusing it. Try fixing the Dial Plan (so these numbers are not categorized as "Unknown" and see if the problem goes away.