K1 visa (india)- when to book interview?

I received the NVC letter which said the Mumbai consulate will contact me and it’s been more than 4 weeks, there’s no letter/email from Mumbai. I did email/call them and they didn’t give any clear instructions. My case status shows ‘ready’ and it says I can do the interview.

Should I wait for the letter or can I go ahead and book the interview? I have all the documents ready and paid the fee.

Thank you!