This past February during Mobile World Congress, WhatsApp CEO Jan Koum revealed plans to offer voice calling in its app by mid-2014. That date came and went with nary a peep on the matter, until now.

During a presentation at the Code/Mobile conference, Koum admitted that building a calling service isn't exactly easy. The development team has had to overcome a number of technical hurdles to bring voice calling to WhatsApp.

One of the issues the executive mentioned is the fact that WhatsApp doesn't have access to all of the microphones on some smartphones. While not a major issue, it does make noise cancellation a tricky proposition. Koum also said they are still trying to figure out how to contend with poor cell reception - an issue that'll no doubt cause problems in developing markets where 2G EDGE networks are still the norm.

The executive clarified that WhatsApp won't compete with Facebook's own Messenger app, primarily because the latter is based on Facebook friends and runs on desktops. WhatsApp, meanwhile, is a pure mobile app.

Koum's voice calling announcement earlier this year took place just days after Facebook agreed to buy the mobile messaging service for $19 billion. The acquisition was completed earlier this month.

At present, WhatsApp has more than 600 million active monthly users with plans to eventually cross the 1 billion active user mark. With financial support and infrastructure from Facebook, Koum said they're now able to focus entirely on growth.