haveibeenpwned.com does not use the k-anonymity method that you've described when searching for phone numbers: https://www.troyhunt.com/the-facebook-phone-numbers-are-now-...
Re:Have I Been Facebooked?
by CloselyChunky on Tuesday April 06, 2021 @10:29AM
(#26712180)
Yeah should have validated that claim first. Seems like the form on hibp.com always submits your input to the server...
Still, if I had to chose between hibf.com and hibp.com, I'd lean to hibp.com since Troy is a known name in the industry and has offered this service for a long time without any complaints.
Edit: I just checked, seems like the form on the frontpage of HIBP also submits your complete email/phone number. Pretty sure I read about how you don't have to submit your personal data to validate against HIBP, not to long ago...
[0]: https://github.com/Fumaz/haveibeenfacebooked-api/blob/master...