Improve the privacy of your contacts by defining which of them should be shared with other apps and which should remain private (secret).
While you may be willing to share most of your phone's contacts with apps like WhatsApp or Instagram, you probably don't want to let them know about some others like your doctor, therapist, etc.
First, you don't communicate with them over WhatsApp anyway. And second, this is a real privacy-issue because the mere fact that you have that number stored means that you are a patient there which is confidential data.
Unfortunately, Android only allows an all-or-nothing approach: either you give an app full access to your phone's contact-list or you don't give it anything.
Use the app "Private Contacts" to store those contacts which you do not want to share. It is mimics the standard functionality of your normal contact app but stores all its contacts separately, not sharing them with any other app.
The app offers a caller-ID functionality to show a notification if one of your private contacts is calling you. This notification will tell you which contact it is that is calling you. The default phone-app would not be able to do so because the contacts are not even shared with that.
Of course, you might say that you don't care about sharing this data. In that case, John Oliver explains far better than me, why you should: https://youtu.be/wqn3gR1WTcA
The app takes the protection of your data seriously and does not send any identifying information to anyone. It is completely open source and can be found on Github under
https://github.com/fgubler/PrivateContacts
### Plans for the future ###
Right now, Private Contacts is completely separated from the "normal" contact list of your phone. All contacts you add in this app, will be private automatically. If you want to create a contact to share with other apps, you need to open the standard contacts app instead.
However, it is still in development. The final goal is to completely replace the standard contacts app. The user will then be able to see both private and shared contacts in the app, allowing him to choose for each contact whether it should be public/shared or private.