l at an even lower cost, since they are packaged as an integral part of the telephone system rather than separate systems. The important thing is to under- stand which functions are possible and to identify those that are important to your organization before you decide upon a system. 98 S The Telecom Handbook CHAPTER 5 Look at the history of the product and find out whether it is con- tinuing to be developed and improved upon by the manufacturer. Whether the Voice Mail is made by the PBX manufacturer or by a different company, it is still necessary to have a sufficient number of connections between the Voice Mail system and the PBX. These may be either analog or digital ports in the PBX, requiring separate circuit boards. You may also install Voice Mail if you use Centrex telephone service where your telephone system switching and other func- tions take place back at the central office of the local telephone company. If you use Centrex service and choose to buy a Voice Mail system housed in your office, you must connect it to the telephone company central office with a circuit known as an SMDI link (station message desk interface). The local telephone companies also sell central office based Voice Mail which may be rented for a monthly fee along with each Centrex telephone number. In this case, the Voice Mail sys- tem resides at the central office and not on your premises. The telephone companies buy their central office based voice mail systems from the same manufacturers as other users. You may want to ask whose system they use and check out the manufac- turer and model before subscribing to their voice mail service. It may seem logical to have one telephone installation com- pany responsible for both your PBX and your Voice Mail. However, many companies elect to have a separate Voice Mail integrator since they want to take advantage of some of the more advanced features often not available with the Voice Mail sys- tems sold by the PBX manufacturers. The integrator typically has more experience and expertise i