How to Receive Phone Calls with the Nexmo Client SDK on Android

In this guide, you'll learn how to receive an incoming phone call in an Android application.

You will create a simple Android app, that will log in a user called Jane. After logging in, Jane is able to receive a call and perform actions such as answer or hangup.

Nexmo Concepts

Before proceeding any further, here are couple of concepts that you'll need to understand.

A Nexmo application allows you to easily use Nexmo products, in this case the Voice API to build voice applications in the Cloud.

A Nexmo application requires two URLs as parameters:

  • answer_url - Nexmo will make a request to this URL as soon as someone makes a call to your Nexmo number. It contains the actions that will happen throughout the call.
  • event_url - Nexmo sends event information asynchronously to this URL when the call status changes; this ultimately depicts the flow of the call.

Both URLs need to return JSON and follow the Nexmo Call Control Object (NCCO) reference. In the example below, you will define an NCCO that reads a predefined text for an incoming call, using the Text to Speech engine.

A Nexmo virtual number will be associated with the app and serve as the "entry point" to it - this is the number you'll call to test the application.

For more information on Nexmo applications please visit the Nexmo API Reference.)

Prerequisites

Application webhook

For your application to connect an incoming phone call to an app user, you'll need to provide a URL as the Answer URL webhook - we've created a gist for you to use.

To add this URL, go to your Nexmo dashboard, navigate to applications, select your application and click the 'Edit' button.

Now, set the application's Answer URL to:

https://gist.githubusercontent.com/NexmoDev/ed91ac99a0b278fbdcbde72ca3599ac7/raw/4a134363f8b3bbebae27f04095a57d0cebc5a1be/ncco.json

and click 'Save changes'.

NOTE: This gist is specific to this tutorial and in a real-life scenario, the answer_url should be provided by a purposely built web solution that can serve custom NCCOs if required.

A Nexmo virtual number Nexmo phone number is a phone number that you can link to your Nexmo Application. When a user calls that phone number, the answer_url that is defined on your Nexmo Application is executed. In this tutorial case, you set up an answer_url in the above step, that upon calling connects the call to your app user Jane.

To link a Nexmo number to your app:

  1. Go to your Nexmo dashboard
  2. Navigate to applications
  3. Select your application
  4. Click the 'Edit' button.

  5. Switch to Numbers tab above, search for a number you'd like, and click Link, to link.

The starter project

Clone or download the GitHub repository on either Kotlin orJava. On that repository you'll find two apps:

  • GetStartedCalls-Start - if you want to follow along and add the code with this tutorials
  • GetStartedCalls-Complete - if you want to look at the final result

Open the NexmoHelper class.

  1. Make sure that in enabledFeatures you have to Features.PHONE_to_IN_APP. You can remove the rest, if you haven't completed their tutorials yet.

  2. Replace the user IDs and tokens:

String USER_ID_JANE = "USR-XXX"; //TODO: replace with the UserId you generated for Jane
String JWT_JANE = "PLACEHOLDER";//TODO: replace with the JWT you generated for Jane
val USER_ID_JANE = "USR-XXX"; //TODO("replace with the UserId you generated for Jane")
val JWT_JANE = "PLACEHOLDER"; //TODO("replace with the JWT you generated for Jane")

Login

To start using the Nexmo Client SDK you need to log in to NexmoClient, using a JWT user token.

In production apps, your server would authenticate the user, and would return a correctly configured JWT to your app.

For testing and getting started purposes, you can use the Nexmo CLI to generate JWTs.

Open LoginActivity. It already has a button handler:onLoginJaneClick(...) that calls loginToSdk(...) method, with the jwt you provided.

When the login is successful, the logged in NexmoUser returns. For convenience, save a reference to NexmoUser on NexmoHelper, and then, start MainActivity.

Complete the loginToSdk() method implementation:

void loginToSdk(String token) {
    NexmoClient.get().login(token, new NexmoRequestListener<NexmoUser>() {

        @Override
        public void onError(NexmoApiError nexmoApiError) {}

        @Override
        public void onSuccess(NexmoUser user) {
            NexmoHelper.user = user;

            Intent intent = new Intent(getBaseContext(), MainActivity.class);
            startActivity(intent);
            finish();
        }
    });
}
fun loginToSdk(token: String) {
        NexmoClient.get().login(token, object : NexmoRequestListener<NexmoUser> {

            override fun onError(nexmoApiError: NexmoApiError) {
                notifyError(nexmoApiError)
            }

            override fun onSuccess(user: NexmoUser) {
                currentUser = user

                val intent = Intent(baseContext, MainActivity::class.java)
                startActivity(intent)
                finish()
            }
        })
    }
}

At this point you should already be able to run the app and see that you can login successfully with the SDK.

Receive incoming call

When the phone number associated with your Nexmo app receives a call, the app should notify the user Jane so that she can decide whether to answer or reject it.

It is done by registering to incoming events, with NexmoIncomingCallListener and implementing onIncomingCall().

For simplicity in this example, you will accept incoming calls only on MainActivity. Open MainActivity and create the NexmoIncomingCallListener to save the reference to the incoming call on NexmoHelper, and start IncomingCallActivity:

NexmoIncomingCallListener incomingCallListener = new NexmoIncomingCallListener() {
    @Override
    public void onIncomingCall(NexmoCall call) {

        NexmoHelper.currentCall = call;
        startActivity(new Intent(MainActivity.this, IncomingCallActivity.class));
    }
};
val incomingCallListener = NexmoIncomingCallListener { call ->
    currentCall = call
    startActivity(Intent(this@MainActivity, IncomingCallActivity::class.java))
}

You need to register and unregister the listener in onCreate() and onDestroy():

@Override
protected void onStart() {
    super.onStart()
    NexmoClient.get().addIncomingCallListener(incomingCallListener);
}

@Override
protected void onStop() {
    NexmoClient.get().removeIncomingCallListeners();
    super.onStop();
}
override fun onStart() {
    super.onStart()
    NexmoClient.get().addIncomingCallListener(incomingCallListener)
}

override fun onStop() {
    NexmoClient.get().removeIncomingCallListeners()
    super.onStop()
}

Answer a call

Once a incoming call is received, it can be answered. Open IncomingCallActivity, and complete the prepared onAnswer() button handler, to start OnCallActivity after a successful answer:

public void onAnswer(View view) {
    NexmoHelper.currentCall.answer(new NexmoRequestListener<NexmoCall>() {
        @Override
        public void onError(NexmoApiError nexmoApiError) { }

        @Override
        public void onSuccess(NexmoCall call) {
            startActivity(new Intent(IncomingCallActivity.this, OnCallActivity.class));
            finish();
        }
    });
}
fun onAnswer(view: View) {
    currentCall?.answer(object : NexmoRequestListener<NexmoCall> {
        override fun onError(nexmoApiError: NexmoApiError) {}

        override fun onSuccess(call: NexmoCall) {
            startActivity(Intent(this@IncomingCallActivity, OnCallActivity::class.java))
            finish()
        }
    })
}

Hangup

The onHangup() handler allows to reject the call. Complete the implementation in IncomingCallActivity to finish the activity:

public void onHangup(View view) {
    NexmoHelper.currentCall.hangup(new NexmoRequestListener<NexmoCall>() {
        @Override
        public void onError(NexmoApiError nexmoApiError) { }

        @Override
        public void onSuccess(NexmoCall call) {
            finish();
        }
    });
}
currentCall?.hangup(object : NexmoRequestListener<NexmoCall> {
    override fun onError(nexmoApiError: NexmoApiError) {}

    override fun onSuccess(call: NexmoCall) {
        startActivity(Intent(this@IncomingCallActivity, OnCallActivity::class.java))
        finish()
    }
})

Register to call status

To be aware of the call status, for example, if one of the members answers or hangs up, you should register to CallEvents. The FinishOnCallEnd is a NexmoCallEventListener that finishes the current activity if the call is completed or canceled.

Register to its instance, to address the use cases mentioned previously.

On both OnCallActivity and IncomingCallActivity, add:

NexmoCallEventListener callEventListener = new FinishOnCallEnd(this);

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    NexmoHelper.currentCall.addCallEventListener(callEventListener);
}


@Override
protected void onDestroy() {
    NexmoHelper.currentCall.removeCallEventListener(callEventListener);
    super.onDestroy();
}
var callEventListener = FinishOnCallEnd(this)

override fun onCreate(savedInstanceState: Bundle?) {
    currentCall?.addCallEventListener(callEventListener)
}


override fun onDestroy() {
    currentCall?.removeCallEventListener(callEventListener)
    super.onDestroy()
}

Handle permissions

For devices running Android 6.0 (API level 23) and higher, creation and operation of calls requires requesting runtime permissions. To simplify the implementation in this tutorial, BaseActivity checks the permissions in every Activity's onStart() and onStop().

To read more about the permissions required, see the setup tutorial.

Conclusion

Congratulations! You have implemented your first Phone to App Voice application with the Nexmo Client SDK for Android.

Run the app on a simulator or a device, and with another device call the Nexmo Number you linked to you application. Then, see that you receive the call, and can answer and hangup.