WhatsApp have announced a new addition to their service, just days after Facebook purchased the message app for US$19 billion.

CEO of WhatsApp Jan Koum told Mobile World Congress that they hope to allow users to make calls by the end of the second quarter of this year.

The overall aim for this project is to expand the company reach to 1 billion users.

This will put further pressure on mobile networks who are already struggling to keep up with the offering on the free-app market.  This does however place a higher value on mobile data within network price plans.

Facebook's purchase of WhatsApp last week caused a massive jump in the number of people using the service.

Koum said that in less than a week it had added a further 15 million active users per month, taking its total number of users to beyond 465 million. 

Earler this week, Facebook CEO Mark Zuckerberg said he has no plans to change how WhatsApp works.

Our coverage of Mobile World Congress comes with thanks to Carphone Warehouse