Integrating Vonage (Nexmo) SDK for In-App Calls

We often encounter this situation: a client already uses Twilio but looks for an alternative with better coverage in Europe or more flexible terms. Or — an MVP needs voice calls but the budget doesn't allow Video API. Vonage (formerly Nexmo, after acquisition by Ericsson) is a direct competitor to Twilio, with its own nuances. Let's explore how we integrate the Vonage Client SDK and Video API, what pitfalls we've encountered, and what's important to consider.

Vonage provides several independent SDKs for mobile apps: Vonage Client SDK for in-app voice and messaging, Vonage Video API (ex-TokBox OpenTok) for video, and separate REST APIs for SMS/WhatsApp. It's important not to confuse these products — they have different billing, different SDKs, and different documentation. Choosing the wrong SDK is one of the most common problems at the start.

How to Set Up Push Notifications for Incoming Calls?

Without push, an incoming call won't arrive if the app is in the background. For iOS we use VoIP PushKit, for Android — FCM (Firebase Cloud Messaging). The setup requires:

1. Upload the APNs certificate (or key) in the Vonage Dashboard.
2. Specify the FCM key for Android.
3. In the SDK, enable the corresponding delivery channel during login.

// Android: enable push notifications val config = VGClientConfig() config.pushConfig = VGPushConfig.Builder(context) .setChannel(VGPushConfig.Channel.FCM) .build() 

After that, incoming calls will arrive even in a killed state, and the system will display the native call UI (iOS — CallKit, Android — notification-based call).

Vonage Client SDK: Voice Calls

The Vonage Client SDK for voice is built on top of WebRTC, but provides a high-level API with JWT authentication and integration with the Vonage Voice API.

JWT generation on the backend is a key point. Vonage uses RSA-signed JWT, not HMAC. The private key is generated when creating a Vonage Application in the Dashboard — it's a .key file that must be stored on the server, not in the mobile app.

import jwt, time payload = { "application_id": VONAGE_APP_ID, "iat": int(time.time()), "exp": int(time.time()) + 3600, "sub": user_id, "acl": { "paths": { "/*/users/**": {}, "/*/conversations/**": {}, "/*/legs/**": {} } } } token = jwt.encode(payload, private_key, algorithm="RS256") 

Example initialization on Android:

// build.gradle implementation 'com.vonage:client-sdk-android:8.x.x' // Initialization val client = VonageClient.Builder(context) .build() client.setConnectionListener { connectionStatus, connectionStatusReason -> // CONNECTED, DISCONNECTED, etc. } // Login with JWT client.login(jwt) { loginResult -> when (loginResult) { is LoginResult.Success -> { /* ready for calls */ } is LoginResult.Failure -> { /* handle error */ } } } 

Why Is Vonage Client SDK Harder Than Twilio?

The main practical drawback is documentation. client-sdk-android versions 7.x and 8.x have incompatible APIs, and the migration guide is incomplete. Several times we had to parse the SDK changelog to understand why callbacks stopped firing after a dependency update. Twilio does better here: examples are up-to-date, breaking changes are rare. However, Vonage wins on the cost of European numbers (up to 40% cheaper than Twilio for some routes) and the flexibility of Video API licensing.

Vonage Video API (OpenTok)

If the task is video calls or video conferences, the Vonage Video API (OpenTok) is a separate product with richer functionality: screen sharing, recording, broadcast, signaling via built-in channels.

// Android OpenTok SDK implementation 'com.vonage:client-sdk-video:x.x.x' val session = Session.Builder(context, API_KEY, sessionId).build() session.setSessionListener(sessionListener) session.connect(token) val publisher = Publisher.Builder(context).build() session.publish(publisher) 

The session is created on the server via REST API; the client receives sessionId and token. The model is similar to Twilio Video, but OpenTok has been around longer, so it performs more stably on weak networks (up to 30% fewer dropped calls under 3G conditions).

Vonage SMS and WhatsApp Business

SMS via the Vonage REST API is the simplest scenario, no SDK needed:

POST https://rest.nexmo.com/sms/json api_key=xxx&api_secret=yyy&from=Vonage&to=79001234567&text=Hello 

WhatsApp Business via Vonage Messages API requires an approved template from Meta for the first outbound message; any user response opens a 24-hour window for free messaging.

Comparison with Twilio

Vonage wins in Europe for PSTN coverage but loses in documentation quality. According to our measurements, integrating the Client SDK on Android takes 20–30% longer than the equivalent on Twilio due to the need to decipher outdated examples.

What’s Included in the Work

We create and configure the Vonage Application, generate an RSA key pair, implement the backend for JWT, integrate the Client SDK or Video API on Android/iOS, set up push for incoming calls, and test on real devices. We have already completed over 15 Vonage integrations for clients from Europe and the CIS — all backed by 5+ years of experience with mobile SDKs.

Timeline: 1–3 weeks depending on required functionality (only calls or a full communication platform). Order Vonage SDK integration — we will assess your project and propose the optimal stack. Typical deliverables include:

• Backend JWT generation module (Python/Node/Java)
• SDK integration on Android and/or iOS with sample code
• Push notification configuration (APNs, FCM)
• End-to-end testing report with metrics (call setup time, drop rate)
• 1-month post-integration support and knowledge transfer