Many methods could be used to reduce the search space, such as not allowing the seventh-from-last and tenth-from-last digit to equal 0. Now I've reduced the search space by two orders of magnitude, so those 300 days just became 3 days.
To be honest, I don't really know the rules governing US phone numbers and just did a cursory google search, which came up with the simple answer of "10 billion".
That is very likely quite an overestimation. But if I'm not mistaken, the limitations you describe only reduce the search space by 19% (since it's two 90%-steps).