All of them. You don't need to provide one on sign up, but your account will be soft banned typically in a couple of hours until you provide one. So it's a requirement that they aren't forthcoming about.

I have an account that doesn’t have one and works fine. I wonder why they don’t always require it.

A year or so ago, I created an account and followed ten or so people (no tweets at that time). When I went to log in the next day, it wouldn't let me log in until I attached a phone number. As I understand it, that was a relatively common occurrence.

And, this is just one of many examples of a deep, deep dishonesty at the core of Twitter Inc's operations:

Pretending they're not requiring something when in practice, a giant proportion of their userbase faces it.

Pretending anything changes when you click 'See This Less Often' on some annoying feature.

Constantly undoing a user's preference for 'Latest' over algorithmic 'Home'.

Claiming they don't "soft-ban" but absolutely, verifiably, hiding some users' content from others who have explicitly followed them.

Implying there's some effective "appeal" process for arbitrary & often clearly erroneous moderations decisions – when instead it's just designed for coercing compliance, including the simualted "voluntary" deletion of tweets, under penalty of losing your account indefinitely.

Slurring & hiding replies with no hint of offense as "potentially offensive".

Describing tweets as "unavailable" when (often) all you have to do is click to see it - wasting users time.

Offering "Show additional replies" even when there's nothing more to show – again wasting users' time.