❓ Frequently Asked Questions

SMS Gateway Master is a professional Android application by Simpapp that serves two purposes:

• Default SMS app — full-featured messaging with Material Design 3, dark mode, smart conversation management, and Room DB-backed local storage.
• SMS Gateway server — exposes a REST API so any application, script, or automation can send SMS programmatically through your Android phone and carrier.

It is available on Google Play: play.google.com → SMS Gateway Master

The flow works in 4 steps:

1. Your app sends an HTTP POST to the Cloud API with your API key and message payload.
2. The Firebase Cloud Function validates the API key and delivers a push notification (FCM) to your Android device.
3. Your Android phone's SMS Gateway Master app receives the notification and uses Android's native SmsManager to send the SMS via your carrier.
4. The app reports the delivery status (sent / delivered / failed) back to your webhook URL.

This local-first architecture means messages route through your own phone and carrier — no third-party SMS cloud intermediary stores or owns your messages.

1. Download SMS Gateway Master from Google Play.
2. Open the app and sign in with your Google account.
3. Navigate to Settings in the app menu.
4. Find the API Key section — tap to generate or copy your unique key.
5. Use the key as the X-API-Key header in every API request.

Security note: Keep your API key secret — anyone who has it can send SMS from your phone.

Endpoint: POST https://us-central1-sms-gateway-ae7e1.cloudfunctions.net/api_sms_send

Required headers:

• X-API-Key: your_api_key_here
• Content-Type: application/json

Request body:

{
  "phoneNumber": "+1234567890",
  "message": "Hello from SMS Gateway Master!",
  "webhookUrl": "https://your-server.com/webhook"   // optional
}

Success response:

{ "success": true, "smsId": "sms_1707400000_abc123", "status": "queued", "timestamp": 1707400000000 }

Full interactive docs: documentation.html — live tester: sms-tester.html

Any language that can make HTTP requests works. Ready-to-use downloadable scripts are provided for:

• 🐍 Python — using the requests library, includes bulk-send loop
• 🟩 Node.js / JavaScript — using native fetch, async/await
• 🐘 PHP — using cURL, no external dependencies
• 📊 Excel VBA — send SMS directly from a spreadsheet cell list
• 📋 Google Sheets — Apps Script with a custom menu button
• 🖥️ Bash / cURL — for Linux, macOS, CI pipelines, cron jobs

Download all scripts at ready-to-use.html.

Webhooks deliver real-time delivery status callbacks to your server. Two modes are supported:

• Default webhook — set once in app Settings; applies automatically to all API calls.
• Per-request webhook — include "webhookUrl" in the request body to override the default for that call only.

When an SMS changes status (queued → sent → delivered / failed), the app POSTs the payload to your webhook endpoint containing: smsId, status, phoneNumber, and timestamp.

Coming soon: Incoming SMS webhooks — every SMS received on your device will be forwarded to your webhook URL in real-time.

The Firebase Cloud Function queues the FCM push notification. When your phone reconnects to the internet, FCM delivers the notification and the app processes the pending SMS send.

Important for OTPs: FCM messages may expire after an extended offline period. For time-sensitive messages, ensure your phone has a stable internet connection. For best reliability: keep the app running in the background and disable battery optimisation for SMS Gateway Master in Android Settings → Apps → Battery.

Plan	Price	Daily API Limit	Duration
Free Trial	$0	100 SMS/day	7 days
PRO Monthly	€5 / month	10,000 SMS/day	Monthly subscription
PRO Annual	€52 / year (save ~13%)	10,000 SMS/day	Annual subscription

Important: The subscription fee is for the app/API service. Actual SMS delivery uses your mobile carrier's plan — you are responsible for any carrier SMS charges. Most plans include SMS at no extra per-message cost.

• Free trial (7 days): 100 SMS per day via API
• PRO plan (€5/month or €52/year): 10,000 SMS per day via API

These limits apply only to API-sent messages. Messages sent manually through the SMS app UI have no artificial limit (subject only to your carrier's standard policies).

SMS Gateway Master is built on a local-first architecture:

• ✅ Message content is NOT stored in the cloud.
• ✅ SMS are delivered via Android's native SmsManager through your own carrier.
• ✅ Your full conversation history is stored locally on-device using Room Database.
• ⚙️ Only routing metadata (API key reference, destination phone number, timestamp) passes through the Firebase Cloud Function — no message body is stored server-side.

You retain full control over your data. No message content is retained by Simpapp's servers.

Sending OTP codes is a primary use case. Make a POST request with your API key, the user's phone number, and the OTP message:

curl -X POST https://us-central1-sms-gateway-ae7e1.cloudfunctions.net/api_sms_send \
  -H "X-API-Key: your_api_key" \
  -H "Content-Type: application/json" \
  -d '{
    "phoneNumber": "+1234567890",
    "message": "Your verification code is: 847291. Expires in 5 minutes.",
    "webhookUrl": "https://your-app.com/sms/delivery-status"
  }'

Include webhookUrl to receive delivery confirmation. Combine with your backend logic to verify the OTP code entered by the user. Typical integration: generate OTP → call API → wait for delivery webhook → compare user input.

Feature	SMS Gateway Master	Twilio	AWS SNS
Cost	€5/month or €52/year	$0.0075+ per SMS	$0.00645+ per SMS
Privacy	Local — your phone	Cloud — their servers	Cloud — their servers
Setup	Install app + get key	Account + compliance + number	AWS account + complex setup
Phone number	Your own SIM	Virtual number (extra cost)	Virtual number (extra cost)
Daily limit	10,000 SMS/day	Unlimited (pay-per-msg)	Unlimited (pay-per-msg)
Scale	Up to 10K/day	Enterprise scale	Enterprise scale

Best for SMS Gateway Master: OTP verification, business notifications, small-to-medium volume, developers who want a flat-rate, private, 5-minute-setup SMS API without per-message billing.

Incoming SMS webhook forwarding is planned (coming soon). When released, every SMS received on your Android device will be instantly forwarded to your configured webhook URL, enabling:

• Two-way SMS conversations
• SMS-based chatbots and automated customer support
• OTP receive and verification systems
• Automated response workflows
• SMS data collection pipelines and CRM updates

Currently, SMS Gateway Master supports outgoing SMS via API with real-time delivery status webhooks.

Complete API documentation is at documentation.html, covering:

• API endpoint details and authentication (X-API-Key)
• Full request/response format and all parameters
• Webhook configuration and payload schema
• Error codes and troubleshooting guide
• Live code examples: cURL, JavaScript, Python, PHP

You can also test the API live at sms-tester.html — send a real SMS from your browser in under 30 seconds.