Notice period buyout

I trying to get a notice period buyout of 30 days because the new company in which I am selected wants me to join in 60 days. I have communicated this clearly in my resignation mail and also told my manager about it. My manager told me to talk to hr however hr is not responding to my calls or message. What can I do in this situation does anyone have any idea how notice period buyouts work please share you thoughts and provide some insights in this matter, thanks in advance