If you're jailbroken, and have a little patience, you can get WhatsApp's new in-beta voice calling feature on your iPhone right now. To do so, you'll need to download the WhatsApp beta, download a package from Cydia, and following a couple of additional steps.
After following this tutorial, you will be able to access voice calling in WhatsApp. I can personally verify that this technique works, and was able to shoot a video walkthrough showing you the steps along with a live phone call.