Number is open all days of the week. You can call them between 9am to 9pm.
Please note that the customer care remains closed on 3 national holidays [Republic Day, Independence Day, Gandhi Jayanti].
Please keep your phones model number and warranty details handy before calling. This will help the CC agent to assist you in a better way.
You can write to HTC customer care via the email link given above. It should open a form in which you can write your query. Please mention the model of your phone and other warranty details if required.