Missed calls: A uniquely Indian idea to save money that became a whole signaling system. A missed call from my driver says: "I've reached". A missed to my driver says: "Bring the car". Now it's evolved into a business estimated at over $100 million. Companies like Zipdial let other companies (or even political parties) allow people to use missed calls, to vote, or sign a petition, or request a free sample. You can even join a political party with a missed call to 07798220033. You'll get a text message confirming your interest, and a call back from the Aam Aadmi Party.
Torch: Nokia designers noticed that Indians tended to use the mobile phone display light to see their way in the dark-for instance with power failures. They had a better idea-and introduced a white LED light to serve as a "torch", in a $30 handset. The idea became popular, and was adopted by other vendors. Some handsets even have a switch for the light. The mobile phone is now the commonest form of a torch in India, a country with frequent power cuts. For higher-end smartphones that have a camera and an LED flash, apps that switch on that LED to use as a torchlight are now popular.
Radio: This is not about using radio in a handset (also a popular feature). There's a unique Indian use of the mobile phone - to replace radio altogether. News radio is highly controlled in India, with no private players allowed. So NGOs unable to set up community radio in rural areas are turning to the mobile. CGNet Swara in Chhatisgarh was one of the earliest, with a server that lets tribals dial in from their mobile phones (or get a callback after a missed call) and upload their "citizen news" reports - and then others call back on their mobile phones and listen to the reports! It's like a little private radio station - all using mobile phones.
A YouTube substitute: Digital Green, an NGO which helps farmers create and share videos using small projectors, also lets them log in to see those videos - for those who have internet access. What about those who don't? A farmer with any cheap mobile phone can call a number, choose a voice menu option (for instance, to listen to a video on irrigation techniques for a maize crop), and then get a callback - which will play back the audio version of that video.
Copier-scanner: A common use of cameras on cheap phones in India is to take quick snapshots of documents. Those filling in an Aadhaar form may want to keep a copy of the form-and there may not be a copier handy. Out comes the mobile phone, a snapshot is taken, and the form submitted. My driver keeps a mobile-phone snapshot of cheques deposited, and for this reason upgraded to a phone with a camera two years ago. Some roadside vendors use their phones to show pictures of items they don't have right then.