Gartner, Inc research has pointed to trends in the UC hosting environments. While many companies still prefer their own on-site implementation within datacenters offering perceived security and control, others are offloading their Unified Communications to the cloud for ease of management and redundant platforms with cost savings. A third option combining the two deployment types known as a Hybrid deployment is also gaining a small amount of ground. Customers must weight geographic redundancy, service availability and cost to determine the correct deployment model they should deploy. While the hybrid model does exist, most companies are choosing either on site or cloud deployment only.

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http://www.planetonetelecomtoolbox.com/blogs/planetone-customer/2014/02/public-vs-private-hosted-uc.aspx