WhatsApp is a great application that is useful for instant messaging from iPhone-to-iPhone for FREE. It is akin to the Blackberry Messenger, and is a rival application to the much famed Ping, which picked up a huge following due to the $0 price tag when it first launched. (They have since launched Ping Lite which is still free).
However, WhatsApp seemingly does not support quite a number of countries, but there is a legitimate way around it. Their website quotes in their FAQ:
What countries do you support?
Most of them! We took extra steps during development to make sure WhatsApp works for almost every country in the world and even among contacts in different countries. However, WhatsApp is currently not supported in Malaysia, India, Indonesia, and Thailand
Step-by-step tutorial on getting WhatsApp working
1. Install WhatsApp from the iTunes Store.
2. Before you launch it, go to Settings, then go to the Phone section.
3. Click on My Number.
4. Key in your number preceded with the + symbol and click Save.
5. Now, launch WhatsApp, and if you haven’t ever launched it before, you will be asked whether or not to allow WhatsApp to send you push notifications. Click OK (or else your functionality will be severely limited. If you previously clicked Don’t Allow, you can go back to Settings to modify it under the Notifications section).
6. You will be asked to key in your phone number again.