The Blackberry software must be 6.0 or later. This feature is only present on these updates. You must be connected to an Internet network. While online on BBM Chat, you will only know if you contact has Blackberry Voice if he/she has an icon on the top-right corner.  Gray-colored icon: the user is offline. Green-colored icon: the user is online. Press the Send button to start a voice chat. To answer calls press the Send button, or select the contact's avatar. You can do anything while using Blackberry Voice Chat. To end a voice chat, press the End button.

Summary:
Check the Blackberry version. Check your Wi-Fi connection. Check if your contact has Blackberry Voice. Start a voice chat. Answer calls. End the voice chat.