Requiring a phone number causes problems and, depending on the situation, is easily bypassed by a determined bot author. [see other comments in this thread] It is also ineffective at preventing people from posting rude, misguided, or hateful comments (people post terrible crap under their own name on Twitter/etc all the time).
Bad actors causing problems on a globally-accessible platform is not a new problem. Multiplayer games from MUDs to the modern MMORPG have several decades of experience with the problem[1]. Most attempts at fixing the problem don't work, because they are don't guarantee the bad actor has to pay what they perceive to be a large cost. Unless the incentives are fixed, bad actors will continue to be a problem.
(I'm summarizing; for as much better explanation, see Raph Koster's talk[1])
Cost does not mean money or anything that is widely available (like a phone number). Credit card numbers are trivial to change, and phone numbers are cheap. The cost to the potential bad actor has to be something they don't want to pay. In an MMORPG, deleting the high-level character that someone spent 20+ hours leveling up hurts a lot more than having to provide a (potentially new) phone or credit card number.
To make a serious punishment like character deletion legitimate, there needs to be some sort of publicly visible due process of law involved before it can be used. That means being informed of the specific accusations instead of Google-style banning for unspecified TOS violations. That means a legitimate appeals process that doesn't simply send appeals to /dev/null or handed to the accuser to judge if the appeal is legitimate (like YouTube's content_id (not DMCA) takedowns).
How could this work on something like Twitter? Banning a troll only forces them to make a new account (a minor expense). Hiding all of their comments for a while (or eventually deleting them if the bad behavior continues) denies the troll the fruits of their misbehavior.
Of course, bots and bad actors is only part of the reason Twitter wants a phone number. Without a Ulysses pact[2] binding their future behavior, Twitter is free to continue acting as a surveillance capitalist[3].
[1] https://www.gdcvault.com/play/1024060/Still-Logged-In-What-A... (everybody developing anything involved (directly or indirectly) with the internet should watch this talk)
[2] https://www.youtube.com/watch?v=zlN6wjeCJYk ( https://en.wikipedia.org/wiki/Ulysses_pact )
[3] http://nymag.com/intelligencer/2019/02/shoshana-zuboff-q-and...