private void callButtonClicked() {
    String userName = mCallName.getText().toString();
    if (userName.isEmpty()) {
        Toast.makeText(this, "Please enter a user to call", Toast.LENGTH_LONG).show();
        return;
    }

    Call call = getSinchServiceInterface().callUserVideo(userName);
    String callId = call.getCallId();

    Intent callScreen = new Intent(this, CallScreenActivity.class);
    callScreen.putExtra(SinchService.CALL_ID, callId);
    startActivity(callScreen);
}