In this chapter, we explored in more detail the new architecture of Unified Messaging in Exchange 2013 and the typical flow of a telephone call. Achieving a high level of availability for an entire UM deployment requires proper planning and consideration. It is not just about Exchange, but all the components that are part of the deployment, from Exchange to SIP peers. By default, most IP PBXs and IP gateways possess good load-balancing capabilities as well as the ability to detect failures out of the box, meaning little or no manual configuration is required. On the Exchange side, the deployment of multiple CASs, DAGs, and the correct configuration of UM IP gateways and dial plans ensure that the service remains available if any of these devices fail.