## Introduction In order to make and receive calls with the BYON API, your application needs to be able to receive network notifications for events like incoming call offers, changes to the status of an active call, requests to place a call on hold, etc. These notifications are sent through a channel your application establishes with the T-Mobile network for each line it wants to make calls on behalf of. ## Establishing and Maintaining the Notification Channel Once the application receives an **access_token**, but before it can initiate, receive, or manage a call, the application must create a notification channel by calling the endpoint **/cpaas/notifications/v1/channels** (see the specification for [BYON Notification Channel APIs](/documentation/bring-your-own-number/byon-api-ref-notif) for more details). ``` curl -X 'POST' 'https://naas.t-mobile.com/cpaas/notifications/v1/channels' -H 'accept: application/json' -H 'X-Authorization: <>' -H 'transactionId: aTransactionId' -H 'Content-Type: application/json' -H 'Authorization: Bearer <>' -d \ '{ "channelType": "Webhook", "channelData": { "webhook": { "notificationUrl": "https://example.com/webhook", "notificationAuthToken": "your_secret_webhook_key" }, "expiresAt": "2025-07-06T12:00:00Z" } } ``` T-Mobile’s service returns a **channelId**, which your application then uses to subscribe to said channel to receive event notifications. To do so, the application needs to call the endpoint **POST /cpaas/notifications/v1/subscribe** (see the specification for [BYON Notification Channel APIs](/documentation/bring-your-own-number/byon-api-ref-notif) for more details). ``` curl -X 'POST' 'https://naas.t-mobile.com/cpaas/notifications/v1/subscribe' -H 'accept: application/json' -H 'X-Authorization: <>' -H 'Content-Type: application/json' -H 'Authorization: Bearer <>' -d \ '{ "channelId": "b78ce7e6c494-4b1d-8675-5309-9697ad18" }' ``` T-Mobile’s service then returns a **subscriptionId** associated with the channelId you created before. With the notification channel created and the subscription activated, the application can then create a WebRTC RACM (short for Registration and Connection Management) session, which registers the line for real-time communication. This is done by calling the endpoint **/cpaas/racm/v1/session** (see the specification for _session-controller_ on the [NaaS-CPaaS Essential APIs](/documentation/bring-your-own-number/byon-api-ref-prereq) page for more details). ``` curl -X 'POST' 'https://naas.t-mobile.com/cpaas/racm/v1/session>' -H 'accept: application/json'-H 'X-Authorization: <>' -H 'transactionId: aTransactionId' -H 'Content-Type: application/json' -H 'Authorization: Bearer <>' -d '{ "deviceId": "urn:uuid:9697ad18-8675-4b1d-5309-b78ce7e6c494", "channelId": "b78ce7e6c494-4b1d-8675-5309-9697ad18" }' ``` T-Mobile returns a **racmSessionId**, a **clientId** (specific to the session and not to be confused with the clientId in the previously mentioned API key pair), as well as the phone number or MSISDN used to identify the registered line. From this point forward, the core T-Mobile phone network will send **virtual voice-over-IP** (VVOIP) call notifications, translating those into event messages sent to the notifications channel created earlier. The developer will poll and pick up these event notifications from the channel and can choose how to notify the application user. In a typical scenario, your application may be running in the background and may trigger a system-wide notification of an incoming call, meant to interrupt a user while playing a full-screen game, consuming a video stream, or otherwise using another application with their device. If the user dismisses the alert, the app can direct the call to terminate and go to voicemail. If the user acknowledges the alert, the application can then connect to the incoming call. The following diagram summarizes the flow for creating a channel for notifications, a subscription, a session, and the notification events that need to be properly handled by your application.

Establishing a Notification Channel