We're going this way as well (requiring & verifying phone numbers, not locking accounts). We don't have a problem with bots, but users in the "not so tech savvy" segment tend to switch/discard/forget their email address. Rather than try to recover their account with us, this group will subsequently just create a new one, and then wonder & complain that their profiles/settings/content/histories aren't carried over. We…
> Turns out that phone numbers, whilst also subject to flux, have better long-term congruence to identity, and thereby help us to detect account duplication and manage it. What service do you offer? Because I would never give a company where I wasn't paying for the service my phone number. How do you guarantee you won't misuse it or have ample protections in case of a breach?
The broader point is that collection of phone number isn't intrinsically a bad thing, it's rather the usage and trust level that matters. Judging by the parameters and caveats in your question, you have a similar perspective.