There are many factors an enterprise and/or service provider must consider when weighing the decision to deploy services in the cloud or on premises, some of these factors include: CAPEX; OPEX; security; cash-flow; technical resources; and time-to-market. A recent report by Diane Myers at Infonetics Research takes an in-depth look at the cost analysis -- the full report can be found at http://connectedplanetonline.com/commentary/hosted-versus-permises-based-092210/.
Many of today's in-demand VoIP applications such as PBX, call center, conferencing, and SIP trunking are good candidates for deployment in the cloud. With the advantages that VoIP offers like reliability, ease of deployment, scalability, feature-set and its connectionless nature these services can be quickly and easily deployed to the enterprise or consumer from the cloud or with on-premises equipment, since they no longer require the same capital or IT investment as with TDM.
Once gain VoIP is all about savings and choice.