A small challenge is that the high-quality carrier lookup APIs charge a fee for each query. The fee is tiny (like 1/10th of a cent per number), but enough to add up.
If anyone is interesting in discussing this, my email address is in my HN profile.
I'd suggest it will depend on goals / scale - if not many are using, probably would be ignored even if bad actors were aware of it. If it started to have real effects, there'd undoubtedly be very intentional efforts to attack it. Beyond just the sporadic script kiddiez / for the lulz set ...
Edit: sorry, "app" - to potentially use the app in some malicious way ... Not sure my comment is so useful, but, I'll leave it since it's unfortunately all too easy to end up with unintended consequences. Though, I favor fighting this garbage wholesale and support any efforts to interfere with the deluge of BS / noise with modern communications tech.
I'd propose that absent specific legal processes contact occurs at the consent and discretion of the contacted. That's one of a number of principles I've been kicking around under the notion of "Communications Autonomy"[1]. There is no fundamental right to attention. As such, any communications system which doesn't provide the ability to manage contact attempts and communications is in violation of those principles, and more concretely, as the annoyance and/or risk factors outweigh advantages and benefits, people and organisations will defect from those systems in droves.
The telecoms industry has been openly expressing concern that trust in the phone network will be lost. And by "phone network" I'm speaking broadly: POTS (plain old telephone service) and PSTN (public switched telephone networks), or any universal direct-access communications system.[2] I think we're seeing that breakdown. A key problem is that there isn't any single successor system that appears ready to step in, and most of the more likely proposed systems entail substantial concerns themselves over monopoly power and abuse, surveillance (state, capitalist, or other-actor varieties), etc.
I've got a few specific suggestions which I'm planning to make in a top-level comment to this thread.
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Notes:
1. See: <https://toot.cat/@dredmorbius/108579251632091173>, <https://toot.cat/@dredmorbius/107742445268072257>, and <https://diaspora.glasswings.com/posts/622677903778013902fd00...>
2. Phone, email, SMS, social networking, postal mail, etc.