Please read up on how the telephone number system works and is implemented.
It is an absolutely terrible idea for any company to horde information, let alone information that is critical and not easily changed.
For botters, they can literally bulk buy numbers for less than $0.5/each, especially in scam rampant countries like India, Turkey, China, Russia... And what about for those with company phones or easy teleco infrastructure access? Way too easy to bypass for actual bad actors.
And have you tried buying a pre-paid sim or getting a new phone number just to discover it is linked to someone else's accounts? You can reset their passwords with the number as a lot of services defaults to relying telco to do their job properly and simply rely on the number. I have bought, used, reset numbers of accounts linked to recycled numbers, services including facebook, whatsapp and even some older banks...

Instead of forcing a telephone number...
Maybe redesign Twitch chat to not rely on ancient IIRC protocol? I know some devs already mentioned it before, if security and botting is a genuinely serious concern, this will resolve majority of the issues without sacrificing viewer privacy.
Just imagine you are a new viewer and you are forced to register a number on the platform, even when the channel explicitly turned it off?
If redesigning the chat system takes too long, why not implementing simple logic such as active chatter accounts with creation day older than several months, being VIP, Modded, granted Editor permissions in affiliate and partner channels. having clips that exceed a x-amount of % of the average viewer in the channel, subbed for x-amount of months / channels...

There is already a verified user option. Like you allow only users who verified their account with their phone or email. You can toggle on and off. For bots following, they are many bots out there, that detects spams and bots right away, and delete their message or bans these accounts. I think SeryBot was one of those.
Hope it could help