Editing Telephone line (section)

== In the United States ==
In 1878, the [[Bell System|Bell Telephone Company]] began using two-wire circuits, called the [[local loop]], from each user's telephone to [[Telephone exchange|end office]]s, which performed any necessary electrical switching to allow voice signals to be transmitted to more distant telephones.

These wires were typically [[copper]], although [[aluminium]] has also been used, and were carried in [[balanced pair]]s of open wire, separated by about 25&nbsp;cm (10″) on [[utility pole|poles]] above the ground, and later as [[twisted pair]] cables. Modern lines may run underground and may carry analog or digital signals to the exchange. They may also have [[Subscriber Loop Carrier|a device that converts]] the [[analog signal]] to digital for [[transmission (telecom)|transmission]] on a [[carrier system]]. Often, the customer end of that wire pair is connected to a [[data access arrangement]],
and the telephone company end of that wire pair is connected to a [[telephone hybrid]].

In most cases, two [[copper wire]]s ([[tip and ring]]) for each telephone line run from a home or other small building to a local [[telephone exchange]]. There is a central [[junction box]] for the building where the wires that go to telephone jacks throughout the building and wires that go to the exchange meet and can be connected in different configurations depending upon the subscribed telephone service. The wires between the junction box and the exchange are known as the [[local loop]], and the network of wires going to an exchange is known as the [[access network]].

The vast majority of houses in the U.S. are wired with 6-position [[modular connector|modular jacks]] with four [[electrical conductor|conductors]] ([[6P4C]]) wired to the house's junction box with copper wires. Those copper wires may be connected back to two telephone overhead lines at the local [[telephone exchange]], thus making those jacks [[RJ11, RJ14, RJ25|RJ14]] jacks. More often, only two of the wires are connected to the exchange as one telephone line, and the others are unconnected.  In that case, the jacks in the house are [[RJ11, RJ14, RJ25|RJ11]].

Older houses often have 4-conductor telephone station cable in the walls color coded with Bell System colors: red, green, yellow, and black as 2-pairs of 22&nbsp;AWG (0.33&nbsp;mm<sup>2</sup>) solid copper; "line 1" uses the red/green pair and "line 2" uses the yellow/black pair.
Inside the walls of the house—between the house's outside junction box and the interior [[wall jack]]s—the most common telephone cable in new houses is [[Category 5 cable]]—4 pairs of 24&nbsp;AWG (0.205&nbsp;mm<sup>2</sup>) solid copper.<ref>Public Service Commission of Wisconsin.[http://www.wwt.net/pdf/telephone-wiring.pdf "Testing, Repairing & Installing Home Telephone Wiring"] {{Webarchive|url=https://web.archive.org/web/20180328173511/http://wwt.net/pdf/telephone-wiring.pdf|date=2018-03-28}}.</ref>

Inside large buildings, and in the outdoor cables that run to the telephone company [[point of presence|POP]], many telephone lines are bundled together in a single cable using the [[25-pair color code]].<ref>{{cite book |last1=Bigelow |first1=Stephen J. |title=Telephone Repair Illustrated |date=1993 |publisher=TAB Books |isbn=978-0-8306-4034-8 |oclc=26632919 }}{{pn|date=February 2025}}</ref> Outside plant cables can have up to 3,600 or 3,800 pairs, used at the entrances of telephone exchanges.<ref>{{cite journal |last1=Wilkens |first1=W.D. |title=Telephone cable: overview and dielectric challenges |journal=IEEE Electrical Insulation Magazine |date=March 1990 |volume=6 |issue=2 |pages=23–28 |doi=10.1109/57.50802 }}</ref><ref>{{Cite book |last=Shoult |first=Anthony |url=https://books.google.com/books?id=or08gGGX_toC&dq=3800+pair+telephone+cable&pg=PA469 |title=Doing Business with Saudi Arabia |date=2006 |publisher=GMB Publishing Ltd |isbn=978-1-905050-67-3 |language=en}}</ref>