Hello Mr__Singh—,
For issues with activating your phone number in iMessage be sure to check the list of carriers for your region here, and see if your current carrier is listed for the region: Wireless carrier support and features for iPhone in the United States and Canada - Apple Support
If the carrier is not listed for your region on the above link then the best next step would be to reach out to your carrier for guidance.
Also double-check to see if you're currently able to send SMS messages successfully in the Messages app (green bubble). If you're unable to send SMS messages, then you'll wish to reach out to your mobile carrier for help.
If the plan from the carrier is a prepaid one, verify you have a current balance available for it. If there isn't a balance then this will also require reaching out to the carrier.
If nothing above has helped, then this page can help reach out to Apple Support to investigate further: Apple Support
Cheers.