Fair points on potentially not knowing the outgoing number. I'll have to chew on that problem for a bit. Difficult, but I don't think unsolvable.
Either way, the solution I proposed is a non-ideal workaround to a better system of truly unique virtual numbers. But as you mentioned, spammers are still gonna spam numbers, whether they're virtual or not.
I think in the grand scheme, I'm ok with the potential of spam as long as no single person (besides the Telecom provider) ever has record of my actual number. If the real number leaks, it would make it easier to transfer the real number, since every existing number you've given out simply redirects to the new number.