Burner numbers are close, but if you use the same one for all your contacts it still becomes an identifier, even if you rotate it. An alternative is a proxy service where you get issued a unique pin for each person-to-person connection, so the same phone number gets used by multiple parties. Those both use the existing phone network in better ways. With a VoIP system you don't need numbers, you just don't want to trade them for usernames or some other global identifier. I implemented one approach with Severus, but I'm also looking for alternatives that increase privacy.