Not correct. When I lived in Long Beach, California, around 1987, the Orange County Transit District ran service into Long Beach but its offices were long distance from Long Beach. I believe the number was (then) 714-547-3311, and from 213-427, for example (it was that far back that Long Beach was still in the 213 area code), was a toll call, they had, for the benefit of people in Long Beach and a few other communities, their "toll-free number" (which, you are correct on that point, was non-dialable, you had to dial operator to get) was the easy to remember Zenith 7-3311 since it matched the last five digits of their number.
Also, for years -- and they may still be using it -- the California Highway Patrol had the statewide number Zenith 1-2000.
So I know of at least two independent cases where Zenith numbers had 5 digits.
I've been living in the Washington, DC area for some 15 years now so I don't know if that is still the case, but it was then.