bots can get phone numbers cheap, a cost of doing business. Speculation: Twitter wants your phone number so they can correlate you with existing marketing data so they can target you more effectively.

I would agree.

Twitter has massive data on your interests but collects only minimal information about you vs, say, Facebook which has your name, birthday etc etc. I would also argue that Twitter has BETTER interest data than Facebook b/c you are constantly interacting with thousands of tweets a day across multiple dimensions. Facebook tends to be your family and a couple pages you happened to like.

If they get your phone number, they get all of the available "data in the cloud" about you e.g. income, purchasing preferences etc. Cross reference the interests with that cloud data and you have a very precise target profile to sell to advertisers.

Twitter doesn't have the purest of intentions in harvesting user phone numbers, that's true. But the flip side of this, especially when I see people getting angry about apps that ask for phone number, is that it's not like 1990's or even 2000's anymore where you wanted to keep your phone number private. It's trivial to get a new phone number now and Signal makes it easy. There's also VOIP services like Google Voice (not sure if Twitter allows the use of VOIP numbers, though).

Because even a phone number is not enough to keep out bots.

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?

Don’t you have to seed the PAYG account with a non-zero amount of money to initially get the number? Even a few dollars multiplied out becomes a significant spend if it’s a unique number per bot.

Totally free and that's just one VoIP provider. https://www.sipgatebasic.co.uk/

its a cat and mouse game. lately I've found many websites show an error when I put in my Google Voice number for sms verification

I don't know how the implementation works, if theres a way to check for virtual vs. real number, but some sms verification service provider seems to have successfully implemented a filter on Google Voice

If Twitter asked for the phone number at registration, they would have fewer signups due to people opting not to complete their registrations. Delaying the ask for a phone number increases the number of accounts they are able to claim.