Can you tell us what you consider a toll free number in the U.S.A.? Where are you calling from? Do you need the country code prefix? do you need to dial '9' first to get an outside line? do you want to allow for the parens that often go around the area code?
What makes a number toll free? From where I sit, any number in the 314 area code is toll free (since that's where I live), but that may not be true for someone else.
My larger point is that the trick to defining the regex is to first understand what it is, EXACTLY, that you want to match. Tell us what, EXACTLY, you think a USA toll free number should look like.
Note that simply giving one example does NOT do that. giving ten examples does not do that. A thousand might, but would also probably still not be complete. think about corner and end cases - those are always the tricky one.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors