it is Highly recommended to upgrade your Firebase Plan to Blaze plan if you have a big number of users . since the Free Plan is Limited. you can see the limits and pricing from here
if you have installed the application and there are no contacts are shown even if you have registered contacts, then you have to make sure that you have deployed Cloud Functions
, after doing that go to Firebase Console > Authentication > Users > and delete all users.
next up you should uninstall the app on your devices and reinstall it again.
you need to do this only once since you did not deploy Cloud functions before registering your users in the app.
Phone Authentication will not work on any Emulator,it's Firebase Limitation
if you have problems when verifying phone number and you are in 'debug' mode,then you have to add your debug SHA1 Fingerprint. by going to Generate SHA1 Fingerprint Tool/OS Type(64 or x86) and run the file debug.bat it will ask you for password enter: android then you have to copy the SHA1 Fingerprint and paste in Firebase Console. like here
if you face a problem when verifying Phone Number 'unknown error occured' make sure that you have added your SHA1 Fingerprint here
if you have issues when deploying or installing npm dependencies please wait for a minute and try again , also try to close the Anti-Virus on your computer since it may block the non-SSL URL's.
you may see some 'warnings' while building your app on Android Studio, please ignore them as they are just warnings and NOT 'errors' , so they have no effect on your app.