Who-called.co.uk is using the ultimate crawl budget optimised website. They noindex all new pages that are generated when a user submits a comment for a phone number.
They only remove the noindex tag if there is a second person submitting another comment on that phone number, or if there is a significant number of searches for the phone number on their website.
They also crawl and scrape phone numbers that have more than 1 comment, from other scam report websites, and they post a made-up comment on that phone number, relevant to the nature of the call, with “index”.
A visibly simple website, with thousands of new pages generated every day by the users and their system, but because of their very clever way of handling their crawl budget, they get hundreds of thousands of daily visitors, and Google almost instantly indexes their newly submitted phone numbers that are set to “index”.