 System Design A typical key system for a small office may be set up in the following manner. When the first line is in use, incoming calls ring on the second line and on down through the sequence of telephone numbers. This is known as the hunt group. It contin- ues until reaching the last line in the group. With all lines in use, the caller hears a busy signal. If the receptionist answers a call, it is put on hold and the called person is told to pick up on line 3. When the button is depressed for the third line at any telephone in the office, the caller is there. Users of the key system press down a key or button of a line to place an outgoing call. They hear the dial tone coming from the local telephone company central office. There is no need to dial 9 to place an outside call. Many key systems are still set up in this manner and are re- ferred to as a square key system (Figure 2.2). This means that every telephone looks the same and picks up the same group of outside lines. This works well in offices where there are no more than ten outside lines. After that, it becomes difficult to remember who is on which line and is confusing for both users and callers left on hold and asked several times, "Who are you holding for?" Figure 2.2 A Square Key System 883 1193 883 1193 883 1193 LINE 3 LINE 3 LINE 3 883 1192 883 1192 883 1192 LINE 2 LINE 2 LINE 2 883 1191 INTCM 1 883 1191 INTCM 2 883 1191 INTCM 3 LINE 1 LINE 1 LINE 1 The Telecom Handbook 35 THE PBX If a company has been operating with a square key system with more than ten lines and it is working well, there may be no need to change if you replace the system. Key systems usually have an intercom SO that the reception- ist can announce a waiting call to the person requested by the caller. The intercom also enables system users to call each other. Key systems typically have more options in terms of internal communications than do PBXs. You may hear that a telephone system is "just a key system," used in a derogatory manner to suggest that it is not as good as PBX