I bring some more technical information:
*IF YOU HAVE THE VERSION 2.11.561 INSTALLED READ FROM THE POINT 7.
1.-Make sure you are using a good antivirus and that it is updated.
2.-Download WhatsApp v560 from the original WhatsApp website. Click here.
3.-Download WhatsApp v561 from here.
4.-Rename the apk files according to your wish so that you can identify the two versions easily. For example, WhatsApp560.apk and WhatsApp561.apk, respectively.
5.-Now copy both the apk files to your smartphone’s storage.
6.-Install WhatsApp v561 by simply tapping on the apk file using any file manager app. Make sure you have ‘Unknown sources’ checked in ‘Settings’ to install apps from third-party sources. The option should pop up on its own if you don’t have it enabled.
7.-Now go to ‘Settings’ and then to ‘Application Manager’ where all your apps are listed/running. Find WhatsApp and ‘Force Stop’ it. This will kill the WhatsApp application running in the background so that WhatsApp can restart again. Don’t worry, your contacts and registration will not be reset. This step is faster than restarting the entire smartphone again, unless you wish to reboot.
8.-Now wait for a WhatsApp enabled caller to call you, or if you know someone who has it, request him/her to call you. Once you receive the call, WhatsApp calling feature is enabled for you.
9.-Now that your WhatsApp Calling feature is enabled, you can go back to step 6, but install the original WhatsApp v560 app which was downloaded from the WhatsApp.com website. This app is the official WhatsApp release and will assure you that you are using original software. The v561 WhatsApp version will be overwritten by the official one.
10.-Continue with step 7 and you are all done. WhatsApp calling should be enabled for you.
Source:
http://www.deccanchronicle.com/150311/technology-mobiles-and-tabs/article/how-get-whatsapp-calling-enabled-your-smartphoneThis is a lot more than receiving a phone call. I certainly wouldn't sideload a modded version of what's app on a phone that had any of my private data on it. Or perhaps I'm misunderstanding something.?