Sinch In-App Calling

Overview

Real-time voice and video SDK for Android, iOS, and JavaScript (Web). Connects to Sinch's cloud for signaling and routing.

Supported call types

• App-to-App (VoIP/WebRTC between users)
• App-to-Phone (call PSTN numbers)
• App-to-SIP (connect to PBXs, contact centers)
• App-to-Conference (multi-party calls)
• Phone-to-App / SIP-to-App (inbound calls)

Agent Instructions

Prerequisites

The user needs a Sinch account with an application key and secret from the Sinch Build Dashboard. See sinch-authentication for credential setup — In-App Calling uses application-scoped auth (Application Key + Application Secret).

Integration workflow

1. Detect the platform from the user's project (language, build system, framework):

   • Android (Kotlin/Java, Gradle) → Read references/android.md
   • iOS (Swift/ObjC, Xcode) → Read references/ios.md
   • JavaScript/Web (npm, browser) → Read references/js.md
   • If unclear, ask the user.

2. Walk through the integration steps in the platform reference. Go step by step — confirm each step is in place before moving to the next.

3. Ask about auth approach: Can the Application Secret be embedded (prototyping only) or must JWTs come from a backend (production)?

4. Ask about call types: Which types does the user need? This determines which sections to cover.

5. For Phone-to-App / SIP-to-App: The user needs a backend ICE callback handler. See the "Phone-to-App / SIP-to-App backend" section below.

SDK Init References

For detailed SDK initialization code per platform:

• Browser: references/sdk-init-in-app-calling-browser.md
• iOS: references/sdk-init-in-app-calling-ios.md
• Android: references/sdk-init-in-app-calling-android.md

Phone-to-App / SIP-to-App backend

Receiving inbound PSTN or SIP calls requires:

1. A Sinch voice number from the Build Dashboard assigned to the app (or SIP origination configured).
2. A callback URL in the app's Voice settings.
3. A backend ICE handler that routes calls via connectMxp:

{
  "action": {
    "name": "connectMxp",
    "destination": {
      "type": "username",
      "endpoint": "target-user-id"
    }
  }
}

Key Concepts

• SinchClient — The core SDK object. Must be initialized with Application Key and started before any calls can be made or received.
• User Identity — A string identifier (e.g., user ID) that uniquely identifies a user in the Sinch system. Set during SinchClient initialization.
• Call Types — App-to-App (VoIP), App-to-Phone (PSTN), App-to-SIP, App-to-Conference, and inbound (Phone-to-App, SIP-to-App).
• Managed Push — Sinch-managed push notifications for incoming calls when the app is backgrounded. Required on all platforms.
• JWT Authentication — Production apps must use backend-generated JWTs (not embedded secrets) for SDK authentication.
• ICE Callback — Incoming Call Event. A backend webhook handler required for Phone-to-App and SIP-to-App calls that routes calls via connectMxp.
• Environment Host — Regional endpoint for the SDK connection (e.g., ocra.api.sinch.com for global routing).

Common Patterns

• App-to-App voice call — Initialize SinchClient with user identity, call callUser("recipient-id"). Both users must have active SinchClient instances.
• App-to-Phone (PSTN) — Call callPhoneNumber("+15551234567") with a CLI (caller ID) set to a Sinch number.
• Receive incoming calls — Register push notifications, implement call listener/delegate, handle onIncomingCall event.
• Phone-to-App routing — Assign a Sinch number to the app, set up backend ICE callback that returns connectMxp action targeting the user.
• Video calling — Use callUserVideo("recipient-id") (or platform equivalent). Requires camera permissions.

Troubleshooting

Symptom	Likely cause	Fix
onClientFailed / clientDidFail	JWT issue — token missing, expired, wrong secret, or malformed	Verify JWT generation: correct app key + secret, kid matches key ID, token not expired. See auth section in platform reference
onClientFailed / clientDidFail	Invalid app key or wrong environment host	Verify key in Dashboard; check environmentHost matches your region
No incoming calls (JS)	Managed push not enabled	Call sinchClient.setSupportManagedPush() before starting — required even for the caller side
No incoming calls (Android)	FCM misconfiguration	Verify FCM credentials in Dashboard ("In-app Voice & Video SDKs" → "Google FCM Identification"); check that the device receives FCM tokens
No incoming calls (iOS)	APNs push not configured or token stale	Verify push certificate/key in Dashboard; ensure registerPushNotificationData is called with a fresh device token
No incoming calls (general)	SinchClient not running on the receiver's device	The receiver's app must have an active, started SinchClient to receive calls. Verify start() completed successfully
App-to-Phone fails immediately	Missing CLI (caller ID)	Set callerIdentifier / cli with a Sinch number
Audio only in foreground (iOS)	CallKit not reporting calls	Report outgoing calls to CallKit for background audio

If the above steps don't resolve the issue, instruct the user to contact Sinch Support with their app key, platform, and a description of the problem.

Public endpoints

Set environmentHost when creating the Sinch client:

Endpoint	Region
ocra.api.sinch.com	Global (auto-routed)
ocra-euc1.api.sinch.com	Europe
ocra-use1.api.sinch.com	North America
ocra-sae1.api.sinch.com	South America
ocra-apse1.api.sinch.com	South East Asia 1
ocra-apse2.api.sinch.com	South East Asia 2

Links

• In-App Calling Overview
• SDK Downloads
• Reference Applications (GitHub)
• Android SDK Reference
• iOS SDK Reference
• JavaScript SDK Reference
• LLMs.txt (full docs index)