An organization that desires to provide employees at one location with telephones has, in general, two options.  First, the organization can acquire one telephone and one telephone line per employee from the telephone company.  Second, the organization can acquire one telephone per employee, a small number of telephone lines from the telephone company, and a piece of equipment that enables the employee's telephones to share the small number of telephone lines.  In general, the second option is substantially less expensive than the first option, and the piece of equipment that enables the employees' telephones to share the small number of telephone lines is called a “private-branch exchange” or “PBX.”  In addition, private-branch exchanges typically provide a variety of useful telecommunications features to their users, such as call forwarding, three-way conferencing, and so forth. 
FIG.  1 depicts a schematic diagram of telecommunications system 100 in the prior art.  Telecommunications system 100 comprises affiliated off-premises telecommunications terminals 101-1 through 101-X, wherein X is a positive integer; unaffiliated off-premises telecommunications terminal 102; affiliated on-premises telecommunications terminals 103-1 through 103-X; private branch exchange telephone system 104; and telecommunications network 105, interconnected as shown. 
The terms “affiliated” and “unaffiliated,” as they apply to the off-premises terminals, refer to whether an off-premises terminal is affiliated with an on-premises terminal (i.e., a terminal served by private branch exchange 104).  The relationship of an on-premises terminal (e.g., terminal 103-1, etc.)  with an affiliated off-premises terminal (e.g., terminal 101-1, etc.)  is described below and with respect to FIG.  2, with regards to extending a received call to one or both terminals. 
Private branch exchange 104 is capable of switching incoming calls from telecommunications network 105 (e.g., the Public Switched Telephone Network, etc.)  via one or more transmission lines to any of on-premises terminals 103-1 through 103-N. Private branch exchange 104 is also capable of handling outgoing calls from any of on-premises terminals 103-1 through 103-N to telecommunications network 105 via one or more transmission lines that connect private branch exchange 104 to telecommunications network 105. 
Private branch exchange 104 is capable of also extending an incoming call to a telephone number in telecommunications network 105, in addition to switching the incoming call to on-premises terminal 103-n, wherein n is an integer between 1 and N, inclusive.  The telephone number that is extended-to in telecommunications network 105 corresponds to an affiliated terminal 101-n. 
FIG.  1 also depicts the address spaces that are relevant to telecommunications network 100 in the prior art.  The term “address space” refers to an addressable region of telephone service.  Address space 111 represents the addressable region served by telecommunications network 105.  Address space 112 represents the addressable region served by private branch exchange 104. 
Private branch exchange 104 acts as a “bridge” between address space 111 and address space 112.  When a calling party places a call to someone served by private branch exchange 104, the calling party uses a dialing sequence that includes a telephone number that belongs to telecommunications network 105 and residing in address space 111.  As part of the dialing sequence, the calling party also uses an extension number that allows access to one of the on-premises telecommunications terminals that reside within address space 112. 
Thus an on-premises telephone number is one that exists within the address space of the private branch exchange, and an off-premises telephone number is one that exists within the address space of the Public Switched Telephone Network. 
Some private branch exchanges enable the user of an on-premises terminal to associate an off-premises terminal's telephone number (e.g., the user's cell phone number, etc.)  with the on-premises terminal's telephone number for features such as automatic call forwarding.  In such instances the off-premises terminal's telephone number is said to be mapped to the on-premises terminal's telephone number.  Typically a private branch exchange that provides such a mapping feature stores the mappings in a table, such as the one shown in FIG.  2.