How many blade servers we can put on 42U cabinets ?
a. Physically usually 4-5 max b. Power requirements for these will most likely require a power distribution unit which can be vertically mounted. c. Make sure the cabinet has sufficent space for good airflow on all sides if it is a enclosed cabinet. Many people purchase cabinets that are exact equipment size and forget that cabling, airflow, and workable space should be a factor in cabinet size selection.

What is the advantage of using blade servers ?
a. Density b. Expanability

Also does this good for VOIP applications ?
a. VOIP applications are independent of blade servers as I believe they are in themselves a separate device.

I have heard there there will be heating issues ?
a. Heating issues are typically caused by improper rack or cabinet setups and lack of sufficient HVAC systems. Incorrect cabinet sizes, lack of cable management, lack of HVAC or airflow systems, and damming or filler plates to direct airflow.