How can we enable voice calling on Whatsapp Messenger for Android devices?

First of all you need to have the latest version of WhatsApp on your device. Update to the latest version if you have not yet done so. There are 3 methods to enable the Voice calling feature.

Official Method

You need to have the latest version of WhatsApp for this to work. Check if you have version 2.11.508 or above, else update.
1. Ask someone who has the feature enabled to call you. WhatsApp has enabled the feature for some users as a beta testing.
2. That's all! Your Voice calling feature is enabled.
Note: You can call anyone without this feature now to enable this on their number.

Terminal Method

Prerequisites
1. You need to have the latest version on your device.
2. Your device must be rooted.
Steps
1. Download and install Terminal Emulator For android from Google play Store.
2. Open Terminal.
3. Type the code su -c am start -n com.whatsapp/com.whatsapp.HomeActivity.
4. Your services will be enabled.
You will not be able to invite someone to enable the voice calling if you follow this method. That means you can only call those contacts which have already have this feature enabled.

Root Explorer Method

Prerequisites
1. Your WhatsApp version should be up from 2.11.508.
2. You need to have a rooted device.
3. An invitation from someone who has this feature enabled.
Steps
1. Download and install Root explorer from Play store.
2. Open Root explorer.
3. Go to /data/data/com.whatsapp/shared_prefs/com.whatsapp_preferences.xml.
4. following lines between
< Type boolean name="call" value="true" />
< string name="call_allowed">all< /string >
5. Save this file.
6. Go to settings>app>whatsapp and Force stop.
7. Run the application again.
8. The feature is now enabled.
The last two methods will not be advisable as they are risky and may stop your WhatsApp to stop working.