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Skype ( /?ska?p/) is a proprietary voice-over-Internet Protocol (VoIP) service and software application. Skype was written by Estonian developers Ahti Heinla, Priit Kasesalu, and Jaan Tallinn, who had also originally developed Kazaa.[9] It has been owned by Microsoft since 2011. The service allows users to communicate with peers by voice using a microphone, video by using a webcam, and instant messaging over the Internet. Phone calls may be placed to recipients on the traditional telephone networks. Calls to other users within the Skype service are free of charge, while calls to landline telephones and mobile phones are charged via a debit-based user account system. Skype has also become popular for its additional features, including file transfer, and videoconferencing. Competitors include SIP and H.323-based services, such as Linphone,[10] as well as the Google Talk service, Mumble and Hall.com. Skype has 663 million registered users as of EOY 2010.[11] The network is operated by Microsoft, which has its Skype division headquarters in Luxembourg. Most of the development team and 44% of the overall employees of the division are situated in Tallinn and Tartu, Estonia.[12][13]