Common Configurations

DID

DID refers to Direct Inward Dialing. From a caller’s point of view, this service is in place if the caller can dial a ten-digit number from the outside and reach an individual without operator assistance. Centrex emulates this capability without any additional configuration; every station has a unique number accessed through Centrex switching.

                If the customer provides their own PBX hardware, activation of DID service requires coordination between the customer and telco. The first step in arranging DID is to reserve the telephone numbers for all employees. For example, a company’s listed telephone number is 555-1234. The telecom manager requests a block of 2,000 DID numbers from the telco. The telco will assign 2,000 DID numbers from 555-2200 through 555-4199. The company pays for these numbers on a monthly basis, but they do not cost as much as actual telephone lines even though each workstation has a unique number.

                Next, the telecom manager must determine how many trunks are required to support the calls from the outside to the company’s employees. With site-based DID, the telco passes the responsibility of handling answer supervision to the customer. Therefore, if an external customer calls Bob and extension 2412, the customer dials 555-2412. The telco CO seizes the next available trunk in the DID group and signals the extension 2412 has a call. DID is most often used to reduce or eliminate the manpower required for a central answering position.

DOD

DOD refers to direct outward dialing. If an employee can dial and reach an outside number without operator assistance, the company has implemented DOD. With modern PBX and Centrex systems, telephone systems can be programmed to route the calls without operator assistance. The term DOD is not used often these days because most companies provide this service.

FX

FX (not to be confused with Fax) is the acronym for Foreign Exchange circuit. Foreign refers to a CO other than the local CO, not a location outside of the country. Companies often use FX lines to provide local numbers that customers can call in cities where the companies do not have offices. For example, a company may be based in New York but receive a large number of calls from customers in Chicago. Calls made from the customer’s location over the FX line emanate from the Chicago (foreign) CO, incurring only local charges for the call from the foreign CO to the called location.

OPX

OPX refers to Off-Premises Extension. An OPX line permits a telephone that is not at a company’s location to function as though it is located at the company’s location. This feature has become popular with the increase of employees telecommuting from home. Outside calls to the employee’s extension number ring at the employee’s home.

TIE Lines

A TIE line refers to a private point-to-point circuit used to connect two voice facilities. TIE stands for Terminal Interface Equipment. For example, a dedicated link between customer PBXs at two different locations would be referred to as a TIE line. Other examples of a TIE line would be between a PBX and Centrex system, or two Centrex systems.