Hi Kiran, you can already require a telephone number, the option is found in the Newsletter Options setting page. You need to enable a Newsletter Subscription Option, which is on the Tracking & Email Capture screen. If you don't use this already then select the Manual option, which allows you to require more fields.

Validating specific country phone number is a bit tricky with the current code base, but it's something I can look into to find a way to do it.