Phone Auth: Custom Domain and *.web.app domain do not issue OTP SMS

[ublanke]

I created a web app on firebase and I am using Phone Auth.

• All domains (.firebaseapp.com, .web.app, customdomain) are listed in Firebase Console Authorised domains.
• I also played with variants in the FirebaseOptions below setting to the according authDomain I want to use.
• (Android and iOS Apps also work)

Regardless of all changes, only .firebaseapp.com and localhost seem to work. So I assume it is some configuration problem.
I can also see from the logs that https://identitytoolkit.googleapis.com/v1/accounts:sendVerificationCode?
sends a token back at all domains. But after that silence.

Running out of options. Any idea what I could have missed to set up? Or more info needed?

static const FirebaseOptions web = FirebaseOptions(
apiKey: 'XXXXXX',
appId: 'XXXXX',
messagingSenderId: 'XXX',
projectId: 'XXXX',
authDomain: 'customdomain.com', // 'project.firebaseapp.com', // 'project.web.app',
storageBucket: 'project.appspot.com',
);