Let me be clear. There is one thing I know fairly well, it is copyright law. I am not a lawyer, however, knowledge of copyright was a constant requirement of my consultancy for 30 years. As an added bonus, I consulted primarily to telecos and often worked with subscriber data and data analysis and presentation of said data for sale and re-use. I am at least, uniquely qualified to answer this question on this forum.
I will explain this the best that I can by: one, defining proprietary verses ordinary means; two, defining the cited case exception and other related copyright considerations; and three, being clear on the answer.
Let me clarify copyright some. The example of a phone book is a misnomer. When you get a telephone, you have entered into a private contract agreement as a private citizen with a private company and the resulting information, made public or not, is private proprietary data and therefore the contents of a phone book is proprietary (pay attention to this word) simply because it cannot be obtained generally through any other means except through company data sources- the subscriber data. If data can be derived through ordinary means, such as walking around and writing down house numbers and street names, then that is publicly available data and clear to use. This is not to say that telephone numbers cannot be obtained through ordinary means. It can be.
To clarify further. To quote from: http://www.lib.umich.edu/copyright/facts-and-data
In no case does copyright protection for an original work of
authorship extend to any idea, procedure, process, system, method of
operation, concept, principle, or discovery, regardless of the form in
which it is described, explained, illustrated, or embodied in such
This paragraph is misleading. This exception described in this paragraph is covered by patent and other laws. Copyright only extends to the creation of a work.
“sweat of the brow” doctrine
...refers to any activity such as going house to house and gathering the data manually. This is the definition of ordinary means. It is possible to knock on doors and ask for the same telephone data. Only in as much as you can gather the facts by ordinary means is that data or portion of the proprietary data public.
The ordinary way around using telephone data is to: one, obtain the original data through legal means; and two, apply the fair use doctrine. This would entail getting a copy of the phone book directly from the company which may be free or for a charge, and organizing the facts within in a different way as to create a new work. Have you tried to get a Seattle phone book when you are in Chicago? You will find that the telephone company will likely charge you a surprising fee for it. However, if you are a telephone subscriber in Seattle and you ask for a Seattle phone book, the fee would be far less or even free. I have had to do this many times. There are people who's job it is just to obtain telephone books from telecos in person and paying the fee if required.
The ruling cited in case Feist Publications v. Rural Telephone in the above link (in this answer) hinges on two facts: one, being that the data by the rural cooperative operator as a local monopoly was required to be made publicly available by operational agreement; and two, that the presentation of the work was copyrighted and not the facts contained within due to fact #1. Therefore, only within narrow parameters can this case be considered as a precedent case and must be discarded. Ordinarily, private company subscriber data is not required by agreement to be made public. You have to remember that rural cooperatives are established as public trusts/entities for the public good and owned by the public and/or cooperative members and therefore operate under legal restrictions that allow it to be approved to operate or exist. Each case is different. The citing of the above case (on the linked page) as an argument without explanation of the carve-out exceptions is misleading.
In the early days of the Bell Telephone company, the company was required as a monopoly to make telephone data public unless restricted by the subscriber. When the Bell company was split into the baby Bells, Bell Atlantic, Bell South, and so on, these companies were still required as monopolies to make telephone data public as defined before. But with deregulation and indeed with VoIP, cellular, and other options, monopolies are rare. Only in monopoly scenarios can the above cited argument be made.
Continuing to cite the link above (in this answer):
Just because data is not protected by copyright, does not mean there
are not other legal considerations that may come into play when you
wish to use someone else’s dataset.
Keep this in mind.
Any given dataset and the presentation there of, regardless of the data origin, is a work unto itself. The public presentation of the facts, irregardless of the means, is a work unto itself.
Given that you are not obtaining the data through ordinary means, even though the data is made public, and regardless of the original origin of the data, it is not free to use as you described and you could be criminally charged and held civilly liable for potential copyright infringement as well for criminal trespass and illicit use of computer and other communications equipment not ordinarily authorized and can fall under RICO statutes.
Is it legal to use? No! Absolutely not! It was not obtained through ordinary means nor is it likely the intent of the website operator to expose proprietary data. Any absence of an AUP (acceptable use policy) will not help you. There are assumptions made under the law as to the "reasonable man", "reasonable standard", and "reasonable assumption" that protects the website owner in this case. It is not reasonable that a clever person would use a "vulnerability in the design/creation" of the website to obtain data for other use. As well, if the site profits from it's activities, further protections come into play.