> it’s reasonable to assume they have a real phone number

Just an anecdote, but I rarely give out my non-Google Voice number on sign-up forms. I don’t want my mobile in a database to be spammed.

If a phone number is required on sign-up, and my Google Voice doesn’t work, I usually pass. Whether for personal or commercial use.

I run a business. Why is it reasonable to assume the use of legacy phone numbers?

We target a particular subset of retail-customer businesses which all generally have a phone-number.

It's not reasonable. Voice is the only phone number I use and I use B2B services. If you prevented me from signing up I'd go to a competitor.

Which mobile carriers do you support, if not Google Fi (which is mine)? Or only landlines (which barely exist at my day job)?

My last position was for a b2b voip telephony replacement type of deal. It would be trivial to load up on real geographic numbers from anywhere in the world and automate it all for bots.