It's a phone number. It's probably been bought / sold a few times already. Unless you're on the level of Edward Snowden, I wouldn't worry about it. But maybe your sense of privacy is more valuable than the outcome you'd get from Claude. That's fine too.

It's my phone number... linked to my Google identity... linked to every submitted user prompt... linked to my source code.

There's also been a spate of AI companies rushing to release products and having "oops" moments where they leaked customer chats or whatever.

They're not run like a FAANG, they don't have the same security pedigree, and they generally don't have any real guarantee of privacy.

So yes, my privacy is more valuable.

Conversely: Why is my non-privacy so valuable to Anthropic? Do they plan on selling my data? Maybe not now... but when funding gets a bit tight? Do they plan on selling my information to the likes of Cambridge Analytica? Not just superficial metadata, but also an AI-summarised history of my questions?

The best thing to do would be not to ask. But they are asking.

Why?

Why only them?

It's an anti abuse method. A valid phone number will always have a cost for spammers/multi accounters to obtain in mass, but will have no cost for the desired user base (the assumption is that every worthwhile user already has a phone).

Captchas are trivially broken and you can get access to millions of residential IP addresses, but phone numbers (especially if you filter out VOIP providers) still have a cost.

Just buy a $5 burner phone number. No need to use your real one.

I pay for a number from voip.ms and use sms forwarding. Its very cheap and it works on telegram as well which seemed fairly strict at detecting most voips.