Its been a month or more since Android users got WhatsApp calling feature. Now the company has officially announced the the calling feature for iPhone which were being Beta tested. The company is now working on Windows platform too and hope that we will get the call feature for Windows in a couple of months.
The new version of WhatsApp for iPhone is v2.12.1. Update the WhatsApp now and you may get calling feature. There is no certainty that calling feature will be enabled because in the print it says update will roll out slowly.
It may take several weeks to get the calling feature. Till then sit back and hope for the best.