Earlier quoted context omitted.
I would guess it started as a reaction to the many botnets used by companies and countries trying to maximize certain opinions On top of the phone number, they also go through purges seemingly once a year, getting rid of up to a million accounts a day. That also doesn't fair well with giving the stock market raw numbers
> reaction to the many botnets Makes no sense. If they wanted to require a phone number to keep out bots, they would simply ask for a phone number at registration. Delaying it like this, claiming “suspicious activity” is hard to see any other way than how I described it.
Some VoIP providers do a PAYG model, so you can choose from any number of phone numbers, regional, local landlines or from mobile numbers for free and only pay when you make a call or answer their answerphone.
Now obviously Twitter will send text messages so you never incur a cost from the VoIP provider.
So how does a phone number solve bot activity?