To contact KLM regarding lost or delayed baggage, the most direct way is to call their dedicated baggage assistance line. For travelers in the United States, you can reach them at 1-800-618-0104 or 877-477-5134. If you are calling internationally, it is often best to use the local customer service number for the country you are currently in, which can be found on the "Contact Us" section of the KLM website. Before calling, ensure you have your Property Irregularity Report (PIR) number, which is a 10-character code (e.g., AMSKL12345) provided to you when you filed the initial claim at the airport. KLM also provides a robust online tracking tool where you can check the status of your bags using your last name and the PIR code. For immediate updates, many passengers find that contacting KLM via social media channels like WhatsApp or Facebook Messenger is a faster alternative to waiting on hold during peak travel periods.