Botched hiring - Avis employé Analyst Booking.com

Absolutely pathetic hiring. I went through the complete hirings process and recieved and offer letter and gave a written confirmation along with joining date and also provided all the documents for background verification. The about 5 days after the confirmation the HR reached out to me and I was unable to pick the call and returned the call later that day which she did not pick up nor return it. After which I was Assuming that things are going well. Meanwhile I had given my resignation in my former company and was serving notice. 15 days later received an email the my application is rejected. Feel like the whole hiring was botched and HR was least bothered to follow up. Absolutely pathetic