How can I receive a notification when the device loses network connectivity?

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

callStateListener = new PhoneStateListener(){
    public void onDataConnectionStateChanged(int state){
        switch(state){
        case TelephonyManager.DATA_DISCONNECTED:
            Log.i("State: ", "Offline");
            // String stateString = "Offline";
            // Toast.makeText(getApplicationContext(),
            // stateString, Toast.LENGTH_LONG).show();
            break;
        case TelephonyManager.DATA_SUSPENDED:
            Log.i("State: ", "IDLE");
            // stateString = "Idle";
            // Toast.makeText(getApplicationContext(),
            // stateString, Toast.LENGTH_LONG).show();
            break;
        }
    }       
};                      
myTelephonyManager.listen(callStateListener,
            PhoneStateListener.LISTEN_DATA_CONNECTION_STATE); 

The accepted answer didn't work for me (my tablet was connected to WiFi - no 3G). The following code worked for me:

public class ConnectivityChangeReceiver extends BroadcastReceiver {

    @Override
    public void onReceive(Context context, Intent intent) {
        System.out.println("*** Action: " + intent.getAction());
        if(intent.getAction().equalsIgnoreCase("android.net.conn.CONNECTIVITY_CHANGE")) {
            Toast.makeText(context, "Connection changed", Toast.LENGTH_SHORT).show();
        }
    }
}

and the change to AndroidManifest.xml

<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />

<receiver android:name=".custom.ConnectivityChangeReceiver">
            <intent-filter>
                <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
            </intent-filter>
</receiver>

You might want to consider using a BroadcastReceiver for ConnectivityManager.CONNECTIVITY_ACTION instead. From the docs:

A change in network connectivity has occurred. A connection has either been established or lost. The NetworkInfo for the affected network is sent as an extra; it should be consulted to see what kind of connectivity event occurred.

This receiver works for both WiFi and cellular data connectivity, unlike PhoneStateListener.LISTEN_DATA_CONNECTION_STATE, which will only notify you for changes in cellular networks.

Not sure if this will help but this looks similar...