. In fact, most of the key systems currently on the market incorporate many, if not all, of the functions of a PBX, SO the distinction has become blurred. You may see the term hybrid referring to a key system that can be set up either like a square key system or a PBX. When selecting a telephone system, it is more important to focus on the system capabilities for features and growth than on whether it is a hybrid key system or a PBX. PBX Design A PBX has traditionally been set up in the following manner. The organization has a main telephone number, sometimes end- ing in a double or triple zero (for example 635-5300). When this telephone number is busy, the calls roll over to the second line and then on through the hunt group as we described with a square key system. With a PBX, there may be a greater number of outside lines and these lines do not appear on every telephone. Instead, they are answered by a central position known as the switchboard or attendant console. The caller tells the switchboard operator or attendant the name of the person called. "May I please speak to Linda Storaekre?" Then the operator extends the call to that person by dialing an extension number assigned to the called person's telephone. To place an outgoing call, the same group of outside lines is used. Since the person placing the call has only an extension 36 so The Telecom Handbook CHAPTER 2 number on his telephone, it is necessary to press the button for that extension number and dial an access code, usually 9. In doing so, a free outside line is selected and the person can then place a call. If all lines are in use, as with a square key system, callers to the main telephone number hear a busy signal. People trying to place outgoing calls, when dialing 9, will also hear a busy signal (sent from the PBX, not the local telephone com- pany central office) indicating that all lines are in use. It is important to understand that the total number of tele- phone calls in progress, whether they are incoming or outgoing, cannot exceed the t