If you provide Waze with your phone number during sign up, Waze will compare this phone number with phone numbers which are stored in the Contact Books of other Waze users who have given Waze access.
If you also provide Waze with access to your Contact Book, you can then each choose to share location and ETA with the other.
You can disable access to your Contact Book as follows:
- in Android: Settings > Apps & notifications > Waze > Permissions > Contacts (toggle off)
- in iOS: Settings > Privacy > Contacts and turn off access to the Contact book.