OpenSky was a service provided by Gizmo5, which is now out of business, to enable communication between users of SIP (Internet standard RFC 3261[1]) and users of Skype (which uses a proprietary protocol).
[2] Early solutions to communication between the two classes of user typically required installation of a gateway.
[3] OpenSky – the first freely available hosted SIP to Skype gateway service[4] – commenced on 9 February 2009.
[5] In March 2009 Skype announced a beta of Skype For SIP, a comparable service for business users.
[6][7] On 30 July 2009 Digium, Inc. announced the opening of Skype for Asterisk Public Beta[8] (prior to the open phase, from 25 September 2008, Skype for Asterisk Beta was apparently non-open[9]).