... appear out-of-sector, though I don't have any strong leads on where that might be.
Another possibility is that the extant PSTN might get its collective shit together. As has been noted (including in comments by @szbalint to this thread) the robocall and other issues plaguing the US aren't evident elsewhere, notably Europe.
Whilst I _do_ think that regulatory reforms might address the immediate problem (my vote: chargeback to the connecting telco for spam calls, with those then ...
Generalistic and moderated instance.