WhatsApp, one of the most popular messaging services on mobile, has long had plans to step up its assault against the likes of Skype and Viber with a free voice calling feature that was initially promised for the second quarter of 2014. Now the company’s CEO has confirmed that the launch is planned for early 2015 instead.

During a presentation at the Code/Mobile conference this week, Jan Koum explained that it was difficult to do voice calling well, and the WhatsApp development team has encountered a number of technical issues that must be ironed out before the feature can be presented to the service’s 600 million monthly active users.

One of those technical issues is that WhatsApp does not have access to all microphones on certain phones, making noise cancellation difficult. In addition, WhatsApp is hoping to find a way to make voice calling operable in areas where data coverage is poor, allowing those in emerging markets where 2G EDGE is still widely used to access the feature.

Most of WhatsApp’s new users are based in markets like Brazil and India, Koum revealed.

Koum hopes that WhatsApp can grow to 1 billion active users within the next few years. The service is now supported by Facebook’s infrastructure and finances following a $21.8 billion acquisition that was completed earlier this month, and remains free to use for the first year, following by a $1 annual subscription fee.

For the time being, that payment model is going nowhere, and WhatsApp has no plans to boost it by introducing ads, which Koum believes would hurt the user experience.

• ViaFierce Wireless