SIM Data Switcher

A minimal Android app that lets you set any active eSIM as your mobile data SIM independently of your voice SIM — no root required.

Built for Samsung Galaxy Z Fold 7 (One UI 8 / Android 16), but works on any Android 8+ dual-SIM device.

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What it does

Opens a screen listing all your active SIMs. Tap one to make it the default data provider. Your voice SIM stays unchanged.

It uses a hidden system API (SubscriptionManager.setDefaultDataSubId) that requires one ADB permission grant — done once, permanently.

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Prerequisites

• A Windows/Mac/Linux PC with ADB installed
• Android Studio (to build the APK), or just install the pre-built debug APK if available
• USB cable (or Wi-Fi ADB)

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Step 1 — Enable Developer Options on your phone

1. Settings → About phone
2. Tap Build number 7 times rapidly
3. Enter your PIN if prompted
4. Go back to Settings → Developer options
5. Enable USB debugging

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Step 2 — Build the APK

Option A — Android Studio

1. Clone this repo:

   git clone https://github.com/QuantDeveloperUSA/sim-data-switcher.git
   

2. Open the folder in Android Studio
3. Click Build → Build Bundle(s) / APK(s) → Build APK(s)
4. APK will be at: app/build/outputs/apk/debug/app-debug.apk

Option B — Command line

git clone https://github.com/QuantDeveloperUSA/sim-data-switcher.git
cd sim-data-switcher
./gradlew assembleDebug          # Mac/Linux
gradlew.bat assembleDebug        # Windows PowerShell

APK output: app/build/outputs/apk/debug/app-debug.apk

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Step 3 — Install the APK on your phone

Connect your phone via USB, accept the "Allow USB debugging" prompt on the phone, then:

adb install app/build/outputs/apk/debug/app-debug.apk

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Step 4 — Grant the permission (one-time, permanent)

This is the critical step. The app needs MODIFY_PHONE_STATE which can only be granted via ADB:

adb shell pm grant com.abfstech.simswitcher android.permission.MODIFY_PHONE_STATE

You only need to do this once. The permission persists across reboots.

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Step 5 — Use the app

1. Open SIM Switcher from your app drawer
2. Your active SIMs are listed (mint, usa urozet, etc.)
3. The currently active data SIM is highlighted in green with a ✓
4. Tap any SIM to switch data to it
5. The screen refreshes immediately confirming the change

Your voice/calls SIM (mint) remains unchanged.

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Troubleshooting

"No active SIMs found" → Make sure the SIMs you want are toggled ON in Settings → Connections → SIM manager

"Error: permission denied" → Re-run the ADB grant command in Step 4. Make sure USB debugging is enabled.

App crashes on launch → Ensure you're running Android 8.0 (API 26) or higher. Check with Settings → About phone → Android version.

ADB not recognized in PowerShell → Download Platform Tools, extract the zip, and run the commands from inside that folder:

cd C:\path\to\platform-tools
.\adb.exe install ...
.\adb.exe shell pm grant ...

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How it works (technical)

Android exposes SubscriptionManager.setDefaultDataSubId(int subId) as a hidden API. It requires the MODIFY_PHONE_STATE permission, which is normally reserved for system/carrier apps. However, Android allows granting it via ADB on debug builds without root.

The app calls this method via reflection, meaning it works without any special build flags.

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Project structure

sim-data-switcher/
├── app/
│   ├── src/main/
│   │   ├── java/com/abfstech/simswitcher/
│   │   │   └── MainActivity.java       ← entire app, ~120 lines
│   │   └── AndroidManifest.xml
│   └── build.gradle
├── build.gradle
├── settings.gradle
└── gradle/wrapper/gradle-wrapper.properties

No dependencies. No XML layouts. Pure Java, single activity.