Calling service Ringo made its debut in India last week, providing a way for customers to make affordable voice calls. While similar services utilize VoIP for calls, Ringo buys minutes in bulk from carriers, which is how it is able to offer local calls for as low as 19 paise/min (less than a cent). To put that in comparison, Airtel charges around ₹1.40 (20 cents) per minute, and if you're on a reduced tariff plan, that comes down to 40paise/min (6 cents). It's clear that carriers are wary of a service like Ringo, and they're reacting to its introduction in the country by blocking it.
Ringo has announced on its website that its service has been blocked by local carriers, and that it will halt domestic operations until it resolves the issue: