diff --git a/documentation/API_documentation/click-to-dial_API.md b/documentation/API_documentation/click-to-dial_API.md
index 59d33d7..b09d638 100644
--- a/documentation/API_documentation/click-to-dial_API.md
+++ b/documentation/API_documentation/click-to-dial_API.md
@@ -177,12 +177,91 @@ Errors follow the standard format:
 
 Reference the [OpenAPI YAML](../../code/API_definitions/click-to-dial.yaml) for exact error codes and descriptions.
 
+#### 4.4.1 422 — Business error codes (Unprocessable Entity)
+
+When a request is syntactically correct but semantically invalid, the API returns `422 Unprocessable Entity` with a business `code` describing the error. The following 422 business error codes are defined in the OpenAPI spec:
+
+| Code | Description |
+| ---- | ----------- |
+| `INVALID_PHONE_NUMBER` | Caller or callee number is not a valid E.164 phone number. |
+| `SAME_CALLER_CALLEE` | Caller and callee cannot be the same number. |
+| `RECORDING_NOT_SUPPORTED` | Recording is not supported for this call. |
+| `CALLER_NOT_AVAILABLE` | Caller number is currently not reachable or not allowed to start a call. |
+| `CALLEE_NOT_AVAILABLE` | Callee number is currently not reachable or not allowed to receive a call. |
+| `INSUFFICIENT_BALANCE` | The account does not have sufficient balance or quota to start this call. |
+| `RESTRICTED_DESTINATION` | The destination number is restricted and cannot be called. |
+
+Example 422 response:
+
+```json
+{
+  "status": 422,
+  "code": "INVALID_PHONE_NUMBER",
+  "message": "Caller or callee number is not a valid E.164 phone number."
+}
+```
+
+Note: Additional common CAMARA error responses may be defined in the `CAMARA_common.yaml` referenced by this API's readiness checklist. Always consult the OpenAPI YAML for the authoritative list.
+
 ## CloudEvent delivery notes
 
 - Providers MUST send status notifications as CloudEvents in structured mode. The HTTP header must be `Content-Type: application/cloudevents+json`.
 - The CloudEvent MUST include the attributes: `id`, `source`, `type`, `specversion`, `time`, and `subject` (subject should identify the resource, e.g. `/calls/{callId}`).
 - The CloudEvent attribute `datacontenttype` MUST be `application/json` for the `data` payload.
 
+The CloudEvent `data` payload for Click-to-Dial `EventCTDStatusChanged` includes `callId`, `caller`, `callee`, `timestamp` and `status` (where `status` is an object with `state` and optional `reason`). Providers MUST set the CloudEvent `subject` to the affected resource (for example `/calls/{callId}`) and set `datacontenttype` to `application/json`.
+
+Example CloudEvent (structured mode) — `CALL_STATUS_CHANGED_EXAMPLE` from the OpenAPI spec:
+
+```json
+{
+  "id": "83a0d986-0866-4f38-b8c0-fc65bfcda452",
+  "source": "https://api.example.com/click-to-dial",
+  "subject": "/calls/123e4567-e89b-12d3-a456-426614174000",
+  "specversion": "1.0",
+  "datacontenttype": "application/json",
+  "type": "org.camaraproject.click-to-dial.v0.status-changed",
+  "time": "2021-12-12T00:00:00Z",
+  "data": {
+    "callId": "123e4567-e89b-12d3-a456-426614174000",
+    "caller": "+12345678",
+    "callee": "+12345678",
+    "status": {
+      "state": "disconnected",
+      "reason": "hangUp"
+    },
+    "recordingResult": "success",
+    "callDuration": "1600",
+    "timestamp": "2017-12-04T18:07:57Z"
+  }
+}
+```
+
+Providers SHOULD ensure event `id` uniqueness and may retry delivery on transient failures. When delivering events to a `sink` that requires authentication, use the provided `sinkCredential` (e.g., set `Authorization: Bearer <accessToken>` for `ACCESSTOKEN`).
+
+## Call flow (sequence diagram)
+
+The following sequence diagram shows the typical flow when creating a Click-to-Dial session, delivering events, and terminating the call. This is a logical flow; implementers may optimize or parallelise steps.
+
+```mermaid
+sequenceDiagram
+  participant Client
+  participant Provider
+  participant Caller
+  participant Callee
+  participant Sink
+
+  Client->>Provider: POST /calls {caller, callee, sink, sinkCredential}
+  Provider->>Caller: Place outbound call to caller
+  Provider->>Callee: Place outbound call to callee
+  Provider-->>Client: 201 Created {callId, status}
+  Provider->>Sink: CloudEvent (status: initiating)
+  Provider->>Sink: CloudEvent (status: connected)
+  Client->>Provider: DELETE /calls/{callId}
+  Provider->>Sink: CloudEvent (status: disconnected)
+```
+
+
 ### 4.5 Policies
 
 N/A