Android App with Audio Calls

I need android developer who has worked with audio calls. We can use Twillio, Agora or any other API that you would need to complete the job.
I have the requirement document and complete mockup designs to make things easier for you.
The app should be built with native android tech stack such as java or kotlin
I will prefer if you go through the document before placing a bid.

Preferred API is now Agora.