Prevent USSD dialog and read USSD response?

It is possible using accessibility service. First create a service class:

public class USSDService extends AccessibilityService {

    public static String TAG = USSDService.class.getSimpleName();

    @Override
    public void onAccessibilityEvent(AccessibilityEvent event) {
        Log.d(TAG, "onAccessibilityEvent");

        AccessibilityNodeInfo source = event.getSource();
        /* if (event.getEventType() == AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED && !event.getClassName().equals("android.app.AlertDialog")) { // android.app.AlertDialog is the standard but not for all phones  */
        if (event.getEventType() == AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED && !String.valueOf(event.getClassName()).contains("AlertDialog")) {
            return;
        }
        if(event.getEventType() == AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED && (source == null || !source.getClassName().equals("android.widget.TextView"))) {
            return;
        }
        if(event.getEventType() == AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED && TextUtils.isEmpty(source.getText())) {
            return;
        }

        List<CharSequence> eventText;

        if(event.getEventType() == AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED) {
            eventText = event.getText();
        } else {
            eventText = Collections.singletonList(source.getText());
        }

        String text = processUSSDText(eventText);

        if( TextUtils.isEmpty(text) ) return;

        // Close dialog
        performGlobalAction(GLOBAL_ACTION_BACK); // This works on 4.1+ only

        Log.d(TAG, text);
        // Handle USSD response here

    }

    private String processUSSDText(List<CharSequence> eventText) {
        for (CharSequence s : eventText) {
            String text = String.valueOf(s);
            // Return text if text is the expected ussd response
            if( true ) {
                return text;
            }
        }
        return null;
    }

    @Override
    public void onInterrupt() {
    }

    @Override
    protected void onServiceConnected() {
        super.onServiceConnected();
        Log.d(TAG, "onServiceConnected");
        AccessibilityServiceInfo info = new AccessibilityServiceInfo();
        info.flags = AccessibilityServiceInfo.DEFAULT;
        info.packageNames = new String[]{"com.android.phone"};
        info.eventTypes = AccessibilityEvent.TYPE_WINDOW_STATE_CHANGED | AccessibilityEvent.TYPE_WINDOW_CONTENT_CHANGED;
        info.feedbackType = AccessibilityServiceInfo.FEEDBACK_GENERIC;
        setServiceInfo(info);
    }
}

Declare it in Android manifest

<service android:name=".USSDService"
    android:permission="android.permission.BIND_ACCESSIBILITY_SERVICE">
        <intent-filter>
            <action android:name="android.accessibilityservice.AccessibilityService" />
        </intent-filter>
        <meta-data android:name="android.accessibilityservice"
            android:resource="@xml/ussd_service" />
 </service>

Create a xml file that describes the accessibility service called ussd_service

<?xml version="1.0" encoding="utf-8"?>
<accessibility-service xmlns:android="http://schemas.android.com/apk/res/android"
android:accessibilityEventTypes="typeWindowStateChanged|typeWindowContentChanged"
android:accessibilityFeedbackType="feedbackGeneric"
android:accessibilityFlags="flagDefault"
android:canRetrieveWindowContent="true"
android:description="@string/accessibility_service_description"
android:notificationTimeout="0"
android:packageNames="com.android.phone" />

That's it. Once app is installed, you have to enable the service in Accessibility Settings (Setting->Accessibility Setting -> YourAppName).

Solution described here and here (russian).

It works in Android 2.3, but i'm not completely sure if this can work in a superior version, follow the instructions:

1. Connect your phone USB to your computer (Debug Mode)
2. Type adb devices (your phone must be listing)
3. Type adb shell
4. Type logcat -v time -b main PhoneUtils:D > output.txt
5. Now in your phone, send a ussd code example: #123# wait a moment and then in your console press Ctrl + C
6. Read the output.txt and find this word displayMMIComplete