Official Facebook Hello contacts/dialer app login only says "104"
"We're moving on" is all Facebook has said since then, adding "We will be deprecating Hello in a few weeks." minus a reason, from an unknown anonymous Facebook employee author.

For some reason Facebook did not tell users who installed the HEAVILY advertised "Hello Dialer" application about (maybe with another ad equally) that they're forgetting the idea now: https://newsroom.fb.com/news/2018/07/hello-tbh-moving-on/.

The reason you did not find information, is that recent news suggests the application in question has been deprecated officially in a quiet press release, in their news article I archived to report at http://web.archive.org/web/20190522194837/https://newsroom.fb.com/news/2018/07/hello-tbh-moving-on because of the secrecy, as that is fairly well hidden for a large organization, and had not been given good meta-data information, to be indexed in searches with priority.