In this Swift adventure series, we built an iOS app using the Twilio Client SDK for iOS that can make outbound phone calls and receive inbound calls. We used the Twilio REST API to bridge any voice connection coming into Twilio to an instance of Twilio Client, allowing us to provide a similar in-call experience to users of traditional telephone numbers. The app accepts, rejects or ignores incoming calls, notifying the user through a UI with buttons for each option. We also implemented the TCDeviceDelegate protocol to notify the app when it receives an incoming call and when it stops listening for incoming connections. Additionally, we set up LocalNotifications to alert the user of incoming calls even when the app is in the background. The final app allows users to be connected to active calls even if the application is running in the background.