Originally, when Skype was launched, it was a proprietary VoIP system. However, Microsoft has made quite enhancements in its back-end infrastructure and it has now completely moved to a cloud-based service powered by Azure. However, Skype for Business can essentially be used and connected with a SIP Trunk to carry outcalls.
Originally, Skype came from a more distributed system. Every device that needed to communicate over Skype had to install a client application due to which it became a node in a network of nodes. The communication was carried out via the Peer-to-Peer network and used the Skype Protocol. Presently, Skype for Business has emerged as a centralized Cloud-based infrastructure in which call management is more controlled and streamlined. Today, Skype can be integrated well into the Business Phone system which includes SIP Phones or VoIP Phones.