This is an anti-spam and anti-abuse requirement, not because they need you to tell them your phone number.
They’re on most of the phones in the world, and have access to all the billing records associated with your phone number, as they’re a cell phone service provider. They’re asking folks who they think might be abusing the service for this information, so they identify themselves in a way that Google can report to law enforcement.
The idea they’d need you to tell them this information so they could use it is kind of laughable.
