Unfortunately, Phone authentication is currently not officially supported by the firebase rest API. (See Firebase Auth Ref ).

I found this blog: Integrate Firebase phone number validation with your backend

I will check this approach if I find the time, but I'm permanently overtasked.

I have investigated more and I see that this function expects that we show the user first a Captcha (probably to reduce the number of such Phone Auth requests). Unfortunately, I did not find enough information to implement such Captcha support.