It's important to allow the Hiya app access to your Contacts: if the Hiya app is not allowed Contacts access, then phone numbers that you wish to receive calls from may be blocked.
To allow Contacts access after you install the Hiya app, open the Hiya app and using the navigation bar at the bottom of the screen, tap on "Protect". On the "Protect" screen you will see a prompt to "Allow contacts access" - tap on this button, then respond "Yes" when prompted to allow Contacts.
If you don't see the Allow contacts access option in the protect tab then you have probably already allowed contacts.
If you wish to disable Contacts access at a later point in time you can do so from the iPhone's Settings app.
*Note that access to Contacts is required for the Enhanced Call Screening feature to function.