To stop it’s postpaid subscriber from churning to other operators and even prepaid service Airtel is offering this as a retention plan for customers who wish to leave Airtel or port for Rs.345 unlimited calls voucher launched recently with 1GB data. 
This Rs349 Myplan infinity is stripped down version which is not listed on Airtel website. Same as prepaid this plan gives unlimited local+std calling + 1GB data however customer get added incoming while roaming free. We are not sure if 100sms are free with Rs.349 plan. Service tax is additional to Rs.349 for postpaid however in prepaid Rs.345 voucher is valid for 28days only not one month as in postpaid.
So it’s for users to compare the plan and opt accordingly whether in postpaid or prepaid.