Why did Zerodha send an email stating the mobile number is blocked?

As per SEBI requirements (PDF), the mobile number linked to the Zerodha account must be updated if it is disconnected. An active mobile number is required to receive communications regarding the Zerodha account from the exchanges and Zerodha. To learn how to update the registered number with Zerodha, see How to change the registered mobile number or email address with Zerodha?