Unable to block text messages

I don;t think you have options to block SMS from advertisers --> Block, filter, and report messages on iPhone - Apple Support (IN)

You may add third part filters for SMS and contact your carrier for reporting this.

This blocking is allowed by the Carriers in India. Example of Airtel (Private) & BSNL (Govt Owned) & Vodaphone is given below

• Airtel: airtel: Prepaid | Postpaid | 4G | DTH Services in India - Airtel
• BSNL: Activate BSNL DND (Do Not Disturb) to Avoid Marketing ...
• Vodaphone: Do Not Disturb Registration Online | DND Activation - Vi