To ensure the authenticity of interactions between your app and Truecaller, you need to generate an app key [ partner key ] from Truecaller developer account by adding your app name, package name and app SHA-1.
To generate a new app key for your Android app, go to the 'MANAGE APPS' section on the developer account dashboard and click on 'CREATE APP'. Select 'Android' in the App type and continue to enter your app details
You package name corresponds to the applicationId in your app level build.gradle file.
You can get to know the SHA1 for your different app builds by following these steps :
Open your project in android studio
Click on gradle menu on the right side and expand it
Click on android and then signing report
You should be able to see the SHA1 fingerprint of your different build configurations [ debug /release ] in the terminal window within android studio.
Once you input your app details and create the app, you will be able to see a unique "appKey" for your app which you need to include in your project to authorise all verification requests.