They already know your phone number anyway. They figure it out by matching your google cookies to your cell geotracking. After a couple days of this data they can infer your number.
Used to be able to create new Google account without a phone number, not anymore?
There is no "real workaround" or google would fix it.
Try a spoof number, using wifi through VPN to spoof IP, must be in airplane mode so cell is off.
https://www.spoofcard.com/Delete cookies, then use the app only in wifi/VPN/airplane with no other google products running. Delete all cookies on the phone immediately before and after app use. I think you're not going to outsmart google demons.
Resistance is futile... Don't fight it