You might be interested to check this demo:
https://github.com/AgoraIO-Usecase/Video-Calling
it shows how to call another user by using agora-rtm-sdk's call invitation api

I had cloned this demo.
I got an identifier code. After clicking the button 'Start Calling Your Friend' ,next page is shown
.
What I do next?
Where I given the phone number?

to call like wats app does not require a phone number. if you want to call a real phone it's a different story, you should use service like PSTN. So what is your requirement? Call a real phone or call someone else via message channel?

I want to call a user using his contact through the app like as whats app voice call. This my requirement. Is it possible by using your sdk?

@AparnaExalture if you both login with our RTM sdk then you can send signaling message to him to start a video call. for contact if you mean real phone number then we do not provide this.

Same. Do you get any solution? I want to implement voice calls like WhatsApp/IMO.