Point is that generally speaking databases rarely delete fields once added, especially field as valuable as a phone number; as such, my assumption is Twitter has had a field in their database for a user’s phone number since it was first released.
Twitter’s been collecting phone numbers since at least late 2006:
So what you are saying is that when they started they required phone numbers? As in, what the commenter I was replying to wrote was not true? The 'footnote' in history seems pretty important when the whole point is that ignoring it leads you to assume things that are wrong.
The part where Twitter was SMS only is like a footnote in its history.