g big, it is often thought to represent many lines, but it does not. The term trunk refers to an outside central office dial tone line that connects to a PBX. A combination trunk (also called a both-way trunk) can be used for both incoming and out- going calls. Callers dialing into the main telephone number of a The Telecom Handbook a 207 OUTSIDE LINES company who are answered by the switchboard may be calling on one of a group of combination trunks. This is set up in what is called a hunting or rollover sequence (also called ISG or Incom- ing Service Grouping.). This means when the first line is busy, the call "rolls over to" the second line, and then on to the third if the second is busy, and SO on until the end of the hunt group. Employees inside the company may place outgoing calls on that same group of combination trunks when they dial 9. Using 9 is a convention, but it is possible to use any number or several num- bers as an access code to place a call through the PBX. Although combination trunks permit both way calling, most telephone sys- tems have the capability to be programmed SO that combination trunks are used for only incoming or only outgoing. You may hear the expression DOD or direct outward dial trunk. This typi- cally refers to a combination trunk used for outgoing calls only. (As with many terms, this one is based upon history. At one time, pre-1970's, it was possible to order a type of trunk from the local telephone company that could only be used for outgoing calls.) Combination trunks are usually ground start trunks. In order for the connection to the telephone company central office to be completed, the switch in the central office must receive a signal from the PBX on your premises. The signal results from a mo- mentary grounding of the circuit. When the signal is received, the connection is completed and you hear an outside dial tone. The ground start trunks provide what is known as supervision. When the trunk is seized by someone dialing 9 the PBX looks ahead to see if the same trunk may