Skype is a software application that allows users to make voice calls and chats over the Internet. Calls to other users within the Skype service are free, while calls to both traditional landline telephones and mobile phones can be made for a fee using a debit-based user account system. Skype has also become popular for its additional features which include instant messaging, file transfer, and video conferencing. Skype has 663 million registered users as of 2010. The network is operated by Skype Limited, which has its headquarters in Luxembourg and is minority owned by eBay. Most of the development team of Skype is situated in Tallinn, Estonia.