I want to subscribe to webhooks event in outlook-calendar integration

Question

{
    "error": {
        "code": "ValidationError",
        "message": "Subscription validation request failed. Notification endpoint must respond with 200 OK to validation request.",
        "innerError": {
            "date": "2024-08-16T05:58:22",
            "request-id": "840bba1d-8a92-48b5-b709-a65ca12a9a44",
            "client-request-id": "840bba1d-8a92-48b5-b709-a65ca12a9a44"
        }
    }
}

I am getting the above error as my notification endpoint is publicly accessible with name: https://x.y.z/en/outlook-calendar/webhooks. I am using php curl library for integration.

What could be the possible error ? I have verified the SSL Certificate of my notification endpoint, it is not expired.

My subscribe request (post) is having payload as :

            'changeType' => 'created,updated,deleted',
            'notificationUrl' => $notificationUrl,
            'resource' => 'me/events',
            'expirationDateTime' => $expirationDateTime,
            'clientState' => '5755-1233-6767-0007',