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.
uVOIPit is a retail international VoIP service and wholesale VoIP termination business. It started in February 2010, and LLC licensed in April 2010. Website: http://www.uvoipit.com