1. Download the app "3cx" from your device's app store
Apple iOS | Android 

2. Open the 3cx app, you will be asked to scan a QR code

3. On another device login to the 3cx web console. Help Article: 3cx - Web Client - Login

4. Select the QR code option in the upper right menu. 

5. Scan the QR code using your app from Step 2

Usage details can be found directly from 3cx. Using the App: iOS | Android