Skype differs from other VoIP-systems by several factors:
Skype is very popular. During the first week in August of 2003, more than 60 thousand people downloaded the Skype-client. Today, Skype is available for the following operating systems: MS Windows, MacOS, IoS, PocketPC, Android and Linux.
Citel Technologies , A Voice over IP Phone company that provides VoIP Business Phone Systems. Its Main products include EXTender, PBXgateway, Upgraded Hardware Products, PBX platforms etc.