A more honest way of saying this would be that it wouldn't have worked -- requiring a phone number during onboarding would drastically lower your conversion rates. So what's changed? Is this a generational (grumble grumble millenial grumble grumble) thing?
Also, I'd love to know what sparked your asking this question. I don't see a particular uptick in services requiring a phone number for signup, which is something I'd have definitely noticed as I don't carry a phone.