The addition of voice calls will add to the success of WhatsApp. Anytime an app adds more features for cheap or free then more users will likely use it. The addition of voice calling will add a value that many applications do not have right now and is needed in the marketplace.
No matter what What'sApp adds to itself it is going to be successful as it is bought by Facebook and will continue to grow as an app and will never see failure because of the company that bought it. Facebook has so many people using it the app will successful without any help.
I think it will help WhatsApp's success quite a bit. As of yet, there isn't a small, easy to use, simple app to replace making phone calls via the carrier's network. Skype is over complicated. Once a really easy to use app comes out to do voice, I think everyone will adopt it very quickly, and WhatsApp has a good chance of being that app.
The addition of voice calls would add to the success of the WhatsApp application. Millions of subscribers are already using the application for text messaging and sending images, but bringing in the ability to do voice calls would make the application more appealing to a wider range of subscribers. Even in today's world there are some people who prefer voice calls to text messages.
I do not believe the addition of voice calls will add tot he success of WhatsApp. I believe WatsApp was purchased by Facebook simply to add to their portfolio and try to hide the fact that they are over evaluated at this point. People have voice calls available to them through their phone service, we don't need more apps that offer this service.