[highlight]UPDATE March 31st, 2015:[/highlight] You no longer need anyone to call you before activating the Whatsapp calling feature. Just install the latest Whatsapp mobile app on your phone and you’re good to go.
You have to activate Whatsapp calling feature on your Android phone if you want to make free voice calls with Whatsapp. To get it, you need to update your WhatsApp app to at least v2.11.561 and after the upgrade, you must receive a voice call from someone who has the WhatsApp calling feature activated. The feature will then be activated once you reboot your phone.
Once activated, you’ll then see three tabs at the top of your WhatsApp interface: calls, chats and contacts. The calls tab will show you a detailed call log and in any chat with a WhatsApp contact you’ll find that the dialer icon at the top now makes calls direct via WhatsApp and not the regular dialer.
The service is free, and not charged by WhatsApp. Much like other apps like Skype and Viber offering VoIP, WhatsApp voice calling uses a data connection to place a call, whether over Wi-Fi or mobile data. This data use however can be chargeable, based on your Internet plan.
Conference calls not supported as at the time of publishing this.
NOTE: You will have to ask your friends if they have activated the feature to make it easier for you get one of them to call you so you can activate yours.