When you create or import a contact, you can choose to share it with your team or not.
If you share it, all the teammates on the Aircall account will see it in their app and be able to call this contact.
If you don't, only you will have the contact in your app.
You can share/unshare a contact at any moment in the Contact view of your app. Hit the Edit button on the contact and change the Sharing switch