🗿 Steintroll

Silently block incoming phone calls by country calling code.

Steintroll is a small, private Android app that screens incoming calls and silently rejects any from country codes you choose — no ring, no vibration, no missed-call notification. It was built to stop a relentless wave of spam calls from a single country that kept rotating phone numbers, where blocking individual numbers was hopeless.

The name is from Norse folklore: trolls turn to stone in sunlight. A spam "troll" call gets caught and petrified — silently rejected.

This is a build-it-yourself app. It is not on the Google Play Store. You compile it from this source and install it on your own device. See Build & install. You are responsible for building, installing, and running it on hardware you own — see Privacy, permissions & disclaimer.

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What it does

• Blocks by country code, not by number. Add +44 (UK) and every call from a UK number is rejected — including new numbers you've never seen. This is what makes it effective against spammers who rotate numbers.
• Two modes:
  • Block these (blocklist) — allow all calls except the country codes you list.
  • Allow only (allowlist) — block all calls except the country codes you list. Useful for, say, an elderly relative who should only receive calls from a few known countries.
• Block withheld / unknown — an independent toggle for private / no-caller-ID calls.
• In-app log of every call it rejected (number, country, time) — review what it caught without cluttering your real call log.
• Scan call history for spam — optionally analyses your call log and suggests foreign country codes that keep calling but you never actually talk to.
• Works when the app is closed. Blocking runs through Android's call-screening framework, which the system starts on demand — you never have to open the app.
• Fully offline. No network, no accounts, no analytics. Everything stays on your device.

Screenshots

Home	Blocked log	Add country	Spam suggestions

How it works

Steintroll registers a CallScreeningService. For each incoming call, Android hands the service the caller's number before it rings. Steintroll resolves the number's country calling code (longest-prefix match), applies a small pure decision function against your settings, and responds with either "allow" or "disallow + reject + skip notification + skip call-log."

It deliberately does not become your default Phone/dialer app — it only takes the call-screening role, which is the least-invasive way to do this.

The blocking logic is a pure, dependency-free function with a full unit-test suite, so its behaviour is verifiable without a device.

For a fuller explanation of the architecture and the reasoning behind it, see docs/design/DESIGN.md.

Build & install

You build the app yourself and install it on your own phone over USB (or Wi-Fi). This needs the standard Android toolchain.

Prerequisites

• Android Studio (latest) — includes the Android SDK and a bundled JDK.
• Android SDK with the Android 16 (API 36) platform and recent build-tools (installed via Android Studio's SDK Manager).
• A device running Android 10 (API 29) or newer — that's the minimum for silent call rejection. (Built and tested on a Pixel 10 Pro / Android 16.)

No separate JDK install is required: the project points command-line Gradle at the JDK bundled with Android Studio.

Option A — Android Studio (easiest)

1. Clone this repo and open the folder in Android Studio.
2. Let it sync Gradle and download dependencies.
3. Enable Developer options + USB debugging on your phone, connect it.
4. Press Run (or Shift+F10). The app installs and launches.

Option B — command line

git clone https://github.com/netspire-no/android-steintroll.git
cd android-steintroll

# Point Gradle at Android Studio's bundled JDK if needed (macOS path shown):
#   echo 'org.gradle.java.home=/Applications/Android Studio.app/Contents/jbr/Contents/Home' >> gradle.properties
# Tell Gradle where your SDK is (Android Studio usually writes this for you):
#   echo 'sdk.dir=/Users/<you>/Library/Android/sdk' > local.properties

# Build and install the debug app onto a connected device:
./gradlew installDebug

Run the test suite with ./gradlew test.

local.properties (your SDK path) and gradle.properties' machine-specific org.gradle.java.home line are environment-specific and not committed.

First run — grant the role

1. Open Steintroll. Tap Enable call blocking.
2. Android shows a system dialog asking which app should screen your calls — choose Steintroll. (Choosing "Phone" or "None" leaves blocking off.)
3. Tap + Add country, pick the country code(s) to block, and you're protected.

The role and your settings persist across reboots; you don't need to keep the app open.

Usage notes

• Country codes aren't 1:1 with countries. +1 covers the US, Canada and much of the Caribbean; +7 covers Russia and Kazakhstan. Blocking is by dial-code prefix, so blocking +1 blocks all of them together.
• Allowlist mode blocks everything not listed — including your own country, unless you add it. (In "Allow only" with just Sweden listed, only Swedish numbers ring.)
• Block withheld is independent of mode — it only affects calls with no caller ID.

Troubleshooting

• Calls still get through. Make sure Steintroll actually holds the call-screening role: open the app — if the "Enable call blocking" banner is showing, the role isn't granted yet. Only one app can hold the role at a time. Also remember blocking is by country code, so the calling number must resolve to a code on your list.
• Stuck on the splash screen (the troll icon) on launch. That screen is Android's cold-start splash, not an in-app loading screen — the app draws its first frame in well under a second on a healthy device. If it lingers, the device itself was likely busy or unstable; reboot the phone and reopen the app, or reinstall (./gradlew installDebug). Your settings live in app storage, so a reinstall over the top keeps them; a full uninstall clears them.
• Allowlist mode blocked a call you wanted. In "Allow only" mode everything is blocked except the codes you list — including your own country. Add the codes you want to receive (e.g. your home country) to the allow list.

Privacy, permissions & disclaimer

• Everything runs locally. Steintroll makes no network requests and collects no data. Your settings and the blocked-call log never leave your device.

• Minimal permissions. It uses only:

  • The call-screening role — to see incoming numbers and reject calls.
  • READ_CALL_LOG — requested only if you tap "Scan call history for spam," and used purely on-device to suggest country codes. Nothing is uploaded.

  That's it. No contacts access, no network, no SMS, and it never becomes your default dialer.

• You build and install this yourself, on hardware you own. It is provided as-is, with no warranty (see LICENSE). Call screening can affect whether you receive legitimate calls; configure it deliberately and verify it behaves the way you expect for your situation. Use of this software, and any calls it allows or blocks, are your responsibility.

Debug companion

Debug builds include a second launcher entry, Steintroll Debug (distinct icon with a red ribbon), that opens a small on-device test console. Because you can't easily place real calls on demand to test a call screener, the console registers a local managed ConnectionService and injects synthetic incoming calls that flow through the real SteintrollCallScreeningService — so you can verify blocking on a physical device without making an actual call. It also shows live status (role, settings) and lets you flip modes and edit the lists.

This console exists only in debug builds — it is not present in release builds.

Test console	Live controls

Verifying blocking with it (one-time setup):

1. In the console's Setup section, tap register test calling account.
2. Enable that account: Settings → Calling accounts → Steintroll Test (Android won't route synthetic calls through it until it's enabled — if test calls seem to do nothing, this is usually why).
3. Make sure Steintroll holds the call-screening role (grant it in the main app).
4. Add a country to the block list in the main app, then tap a quick call button (e.g. 🇬🇧 +44) in the console. A blocked call is silently rejected and appears in the in-app log; an allowed number rings through.

The test account and (after a full uninstall) your settings are cleared on reinstall, so you may need to repeat steps 1–2 and re-add your countries after reinstalling.

Tech

Kotlin · Jetpack Compose · Material 3 · Room · DataStore · CallScreeningService. Min SDK 29, target SDK 36. No third-party runtime dependencies beyond AndroidX.

License

MIT © 2026 Netspire

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_{🗿 Built by Netspire — for humans, against trolls.}