curl --request GET \
  --url https://{defaultHost}/v1/apps/{app_id}/message_service_providers \
  --header 'Authorization: Bearer <token>' \
  --header 'X-Authorization: <api-key>'

{
  "message_service_providers": [
    {
      "provider_type": "<string>",
      "config": {},
      "enabled": true,
      "name": "<string>"
    }
  ]
}