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Imagine you want to use your cell phone to, say, order take-out food or chat with a friend, but you don’t want to pay for making the call, or the text message. The answer to that ultra-low-cost question is India’s fascinating growth of the “missed call” ecosystem, where callers who aren’t willing to spend on, well, really anything, use the “ring once, hang up” to signal to commerce companies and friends alike on the receiving end that they want to communicate with them.

It’s essentially the poor man’s text message: a free way to nudge another person or company, but which comes in just one flavor. Indian cell phone subscribers, of which there are 900 million accounts, have a monthly average revenue per user of $3, which is rock bottom low for even a developing market.

But an indicator the missed system has gone fully mainstream is that missed calls are increasingly being used as the basis of entrepreneurial ventures. Google (s GOOG) India’s Managing Director Rajan Anandan called “India’s missed call culture” a “massive phenomenon” at an event on Monday night organized by the group Geeks on a Plane Delhi, which I’ve been traveling with this week.

Missed calls are being incorporated into mobile apps and services as a standard type of messaging like a text or an answered call itself. For example, an Indian cloud telephony service provider startup called KooKoo has been working with a Bangalore-based company to create an information market based around missed calls. If you want to know the latest weather, the latest Groupon-style (s grpn) deal, or the real-time bus schedule, you can send a missed call to the designated number and get an automated or manual voice call back with the answers you need.

While the missed call system is increasingly fodder for entrepreneurs, the phenomenon has been a slight problem for the cell phones companies in India, as the free service seems to make up a significant portion of cellular traffic. According to a study from the Learning Initiatives on Reforms for Network Economies (Lirne) a couple of years ago, over half of Indian cellular subscribers made missed calls to convey a message.

The 600 million cell phone users who have 900 million cell phone accounts in India (many people have two phones) are fundamentally fueled by the search for extremely low prices in many cases. There are a lot of interesting reward programs that could be compelling, and I mentioned a poop-fueled new company earlier this week.

What do you think? Are you using missed calls for an ad-hoc low-cost messaging system?