Hitching a free ride on mobile services ( the logos of OTT messaging apps in India are not part of the original illustration which we carry with permission from VisionMobile)
They're going over the top with mobile messaging!

Third party messaging  apps  have overtaken SMS  on phones  and -- for now  -- they're free!
Bangalore, August 25, 2014
By Vishnu Anand
That earthy American saying: "There's no such thing  as  a free lunch!" seems to have met its  match --  on the mobile  phone.  Here it seems, one can, indeed,  get something for nothing -- free  messaging apps that  ride on the Internet and  bypass the paid SMS gateways.  Technically they are what are called Over The Top  (OTT messaging where a third party provides instant messaging services as an alternative to text messaging services provided by the network provider. As long as you can  latch on to the Internet -- at a WiFi hot spot, say -- you have free and unlimited messaging.
The biggie in this space -- WhatsApp with some 500 million users --  has been acquired by FaceBook for $ 29 billion  and will likely become its signature messaging  tool. Phone users lapped up such  apps, sending some 20 billion  free messages last year, racing past 17 billion SMS messages.
Let’s take a look at some of these free apps that are finding  a huge fan base among smart phone owners in this country:
WhatsApp: With its automatic synchronization with numbers on your phonebook (other apps followed this technique successfully), you are forever ‘online’ and connected. With group chat and location sharing capabilities, WhatsApp has gone on to play the role of ‘habit building’ for OTTs
WeChat: The only thing that WhatsApp initially lacked was fulfilled by WeChat – a sticker gallery that allows users to pick and choose animated emoticons to express themselves better. WeChat has gone the Bollywood way through endorsements, movie sponsorships and tie-ups. (www.wechat.com)
Viber: A slightly late entrant in this space, Viber had to compete in a market space where free messaging, free voice calling and free image sharing were  already on  offer. The app promises ‘best quality’ voice calls and a ‘no registration’ and ‘no invite’ policy making it conveniently anonymous. (www.viber.com)
Line: This app takes the game to a whole new level, allowing users to create a ‘snap movie’ – by mixing images, sounds and backgrounds – your very own mini movie that can be shared with your friends...the very same friends that have added you by shaking their phones, another of Line’s innovations where two phones with the app installed in them can add each other’s contacts by simply shaking the phones at the same time.(www.line.me)
Hike:  This is the only 'made in India' messaging app  in our list. With over 20 Million users over 80% under the age of 25, it offers features like "Hidden Mode", the ability to  hide ones private chats and "Hike Offline", the ability to send messages to friends who may be offline at the moment. ( www.hike.in)
Indian Mobile providers are complaining to government that such free messaging  has cut their own revenue by Rs 5000 crores a year  -- but as of now there are no moves to  levy a charge. On the contrary,  some telecom companies    seem to have accepted the reality:  Airtel is bundling  Hike; Reliance is offering WhatsApp as part of  its  own package.   If you can't fight 'em, join 'em!

More reading:  Keen to learn why  OTT is being called a "second tidal wave of mobile ecosystems ready to thoroughly reshuffle the mobile market once more"? This two-part blog by Stijn Schuermans  at the portal of VisionMobile, leading research compaany in the app economy,  might help:
OTT2: the second tidal wave of mobile ecosystems
The surprising business model of OTT2 messaging apps