Start with user requirements for the system
How many people will use email or phones? How many mailboxes will the company need? How many messages will people receive per day? What is the average message size? What is the average number of phone minutes used per day and bandwidth per call? What kind of availability is expected? How about security? System vendors can provide additional direction about relevant user requirements. If an existing phone or email system is being used, there may be tools available that can measure and report current usage patterns to help answer these questions.
Second, translate user requirements into specifications for servers, storage and network equipment. Here again, vendors can help by offering configuration guidance and sizing calculators. The output will specify CPU performance, memory, storage capacity and IOPS, network bandwidth and ports. Be sure to add a buffer for future expansion and growth. If high availability is required, you may opt for a failover site with redundant hardware and data replication.
Third, select hardware that meets the specifications and calculate its physical size and power. Depending on your architecture preferences, you may choose scale-out rack servers, scale-up enterprise servers or converged infrastructure. Virtualization can be used to consolidate more applications and server roles onto it. After selecting the hardware, you can determine its size in rack units and power consumption.