Hello friend's
If you want to create a Call State function in Android see the below code example. Here I have used TelephonyManger method to get the information of call state And I have used PhoneStateListener. In AndroidManifest.xml I have added Phone state permission and sdk version.

Step(1)-MainActivity-

     public class MainActivity extends Activity {

            @Override
            protected void onCreate(Bundle savedInstanceState) {
                super.onCreate(savedInstanceState);
                setContentView(R.layout.activity_main);

                TelephonyManager telephonyManager =
                        (TelephonyManager)getSystemService(Context.TELEPHONY_SERVICE);

                PhoneStateListener callStateListener = new PhoneStateListener() {
                    public void onCallStateChanged(int state, String incomingNumber)
                    {
                        if(state==TelephonyManager.CALL_STATE_RINGING){
                            Toast.makeText(getApplicationContext(), "Phone Is Riging",
                                    Toast.LENGTH_LONG).show();
                        }
                        if(state==TelephonyManager.CALL_STATE_OFFHOOK){
                            Toast.makeText(getApplicationContext(),"Phone is Currently in A call",
                                    Toast.LENGTH_LONG).show();
                        }

                        if(state==TelephonyManager.CALL_STATE_IDLE){
                            Toast.makeText(getApplicationContext(),"phone is neither ringing nor in a call",
                                    Toast.LENGTH_LONG).show();
                        }
                    }
                };
                telephonyManager.listen(callStateListener,PhoneStateListener.LISTEN_CALL_STATE);

            }

            @Override
            public boolean onCreateOptionsMenu(Menu menu) {
                // Inflate the menu; this adds items to the action bar if it is present.
                getMenuInflater().inflate(R.menu.menu_main, menu);
                return true;
            }

        }

Step(2)- Android manifest.xml-

  <?xml version="1.0" encoding="utf-8"?>
    <manifest xmlns:android="http://schemas.android.com/apk/res/android"
        package="com.tiwari.rajshekhar.callstate"
        android:versionCode="1"
        android:versionName="1.0">
        <uses-sdk
            android:minSdkVersion="8"
            android:targetSdkVersion="17"/>
        <uses-permission android:name="android.permission.READ_PHONE_STATE"/>

        <application
            android:allowBackup="true"
            android:icon="@mipmap/ic_launcher"
            android:label="@string/app_name"
            android:supportsRtl="true"
            android:theme="@style/AppTheme">
            <activity
                android:name=".MainActivity"
                android:label="@string/app_name"
                android:theme="@style/AppTheme.NoActionBar">
                <intent-filter>
                    <action android:name="android.intent.action.MAIN" />

                    <category android:name="android.intent.category.LAUNCHER" />
                </intent-filter>
            </activity>
        </application>

    </manifest>