Twitter doesn't have the purest of intentions in harvesting user phone numbers, that's true. But the flip side of this, especially when I see people getting angry about apps that ask for phone number, is that it's not like 1990's or even 2000's anymore where you wanted to keep your phone number private. It's trivial to get a new phone number now and Signal makes it easy. There's also VOIP services like Google Voice (not sure if Twitter allows the use of VOIP numbers, though).