Joining date extention

Hi all,

I have received Infosys offer letter today , where itnwas mentioned that my joining date would be 30th December 2024.I haven't resigned officially yet and with respect to 90day notice period policy I couldn't join on 30th December.I have mailed the same to Infosys HR and tried to call her but I didn't get any response.Please suggest what needs to be done in this case.TIA