How to use Whatsapp

Whatsapp is a messaging app based on the phone numbers in your contact list. It does not use sms or mms but uses internet data, so that you don't need to spend on sms. It works on all smartphone platforms (except obscure ones) including Blackberry (it runs fine without BIS too sometimes), Android, iPhone, Window Phone, Nokia Symbian, and also Nokia s40 (that includes all asha phones, individual model no.s can be found on the Whatsapp website)

1. Install the app from http://www.whatsapp.com/download/ on your mobile phone.
2. Give it required permissions to access internet via cellular (EDGE, 3G) data, wifi and to contacts on your phone.
3. While installing, it will send a local sms to the phone number that you are registering with, to make sure that the number is being used in the handset that Whatsapp is being registered on. So you need to make sure you can send and receive sms on the phone that you are installing it in. It will automatically read the incoming message - that message has a code that Whatsapp will use to register.
4. Once registered, you will see the app interface in the top row which shows current 'chats' (which should be empty if you are using it for the first time), 'favourites' which is essentially all of your contacts who are using whatsapp too, 'all contacts' is the contacts in your phone's addressbook, "Status" where you can set you status message - you can have custom status messages too, and Settings where you can decide how the app behaves.
5. To start chatting, press the 'new chat' button, select the contact you want to say hi to, and type 'hi' and press send. Another way to start is to select your contact from the favourites menu and start chatting. If you do not find that contact in the favourite menu, try the search function (in the blackberry, simply start typing the name of the contact in the favourites menu). If you still don't find them it means that they do not use whatsapp, or they have registered a different number (which is not present in your addressbook). You have the option of searching them up in 'all contacts' and inviting them via normal/standard SMS.
6. You can also form groups in whatsapp, Simply start a 'new group chat', select the contacts you want to add to the group and you are good to go. You can also add new members to the group later on.

Using whatsapp with a different number:
Once installed and registered, You can change the sim card in the phone to keep using whatsapp with the old number, provided you don't need to re-register on whatsapp. You can use this while roaming internationally and use the local sim to make call while still being accessible on whatsapp using your old number.

If you are changing your number permanently, you can still continue the same conversations on your new number by going to 'settings' and then 'account' and then look for an option to 'change my number'. This will make sure that your group chats and other conversations continue. But for the contacts whom you have not initiated chats with, they will see your number as an unknown number unless they have it their phonebook. So best to send them a message saying that this is your new number.

Voice messaging:
You can send voice notes to your contacts too. It might be fun thing to do or you can use it if your hands are not free or you don't have time. In touchscreen phones, there will be a voice chat icon next to the chat box or if you are using a qwerty it can be activated by pressing and holding the spacebar, recording you message and releasing the spacebar to send the message.

I'll be posting a 'how to" for BBM once it releases on iOS and Android.