How Two-line Phones Work


Is a family member hogging the phone? You can solve the problem with a two-line phone.
Is a family member hogging the phone? You can solve the problem with a two-line phone.
Bob Elsdale/Getty Images

If you grew up in a household with a family member who monopolized the phone, then you don't need someone to sell you on the wonders of having multiple phone lines. Beyond not having to worry about the queen of gab tying up the line, two-line phones can enable someone working from home to have a separate line dedicated to business calls.

Business travelers can't live without the things. Two-line phones are a standard feature in most hotels, alongside Internet access, hookups for your MP3 player and ergonomic chairs. The phones also allow dial-up Internet users to surf the Web and catch up on gossip simultaneously. Two-line phones incorporate two separate phone lines (two different phone numbers) into one device.

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If you own one of these phones, you may have wondered how two separate phone lines could originate from one device. After all, single-line and two-line phones look remarkably alike, and they plug in the same way. Where's the magic?

Pull the phone cord out of the jack and look at it closely. You'll probably see four wires inside. Some phone cords have more and some less, but the standard number is four. If this is the case with your cord, then surprise, your business or residence is ready for two lines. It turns out that one phone line only requires two of those wires, so you have two to spare. To activate the second line, all you have to do is contact the phone company and ask a representative to enable the other two wires for your second line.

In this article, you'll learn how callers can make use of two phone lines on the same phone. The next time your other line rings in the middle of your family member's marathon discussion, you'll know a little more about the mechanics of what's going on. Next, find out how two-line phones operate so you don't miss a single call.

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Connecting a Two-line Phone

Two-line phone splitters are an alternative to two-line phones. They work by diverting one of the lines to a first single-line phone and the other line to a second single-line phone.
Two-line phone splitters are an alternative to two-line phones. They work by diverting one of the lines to a first single-line phone and the other line to a second single-line phone.
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Two-line phone options are seemingly limitless. Retailers advertise a plethora of phones with multiline capabilities. There are two-line business phones, two-line touch phones and two-line cordless phones. Two-line cell phones are even an option now.

All telephones have a jack where you plug in the phone cord. The phone jack is connected to the wall jack, which connects to the telephone company's wiring. The phone jack is lined with contacts, or conductors, that recognize the wires inside the cord and establish a connection with the line. Whereas the contacts in a two-line phone jack automatically recognize all four wires, and thus both phone lines, the contacts in a single-line phone jack only recognize the first line. So even if you have two phone lines up and running, if you plug the two-line cord into a single-line phone jack, you'll still only receive calls from the first line.

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Two-line phones instantly recognize both phone lines and detect which one is ringing. It's possible to use two phone lines without using a two line phone, but you have to either rewire your wall jacks or purchase a two-line splitter. These devices plug into a standard two-line jack and split the line, directing the first line to one jack and the second line to the other jack. With a splitter, you'll just have two single-line phones coming off the same jack. One of the phones will receive calls from line one, and the other will receive calls from line two.

Ultimately, two line phones aren't that different from their single-line counterparts. Take a look inside these double-duty devices on the next page.

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Two-line Phone Mechanics

Two-line phones usually have a jack for line one and a jack for line two, plus a combination jack that can detect both lines.
Two-line phones usually have a jack for line one and a jack for line two, plus a combination jack that can detect both lines.
Stockbyte/Getty Images

If you've done your homework, then you already have a good understanding of how single-line phones work from the HowStuffWorks article How Telephones Work. Whereas single-line phones have just one jack, two line phones have either two or three. If your telephone lines aren't combined into one cord, you plug the first line in the first jack and the second line in the second jack. If the lines are combined, you just plug the single cord into the third jack, which is designed to detect all four wires.

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At the simplest level, two-line phones establish a connection the same way single-line phones do; they just have two of everything. The conductors in the phone jack detect whether a first and second land line exist and then assign each line to the appropriate user interface. Line one functions are controlled by one interface, and line two functions by another. Condition detectors inside the phone recognize if the phone lines are connected to landlines and then operate accordingly. These detectors select one of the two phone lines for use when they check for predetermined conditions, such as the particular line's phone number. Two line phones also use opto-isolator means, which isolate the line circuits and prevent cross talk.

Two-line phones can operate flexibly depending on how the phone lines are connected. Sometimes just line one may be in use, sometimes just line two and sometimes both. A switching mechanism inside the phone detects when one of the lines is in use and causes a light on the phone to light up to signal that the line is being used. Depending on the phone model, several other switching mechanisms may exist for the implementation of functions like hold, intercom and conference calling. Conference calls enable the telephone user, the person on line one and the person on line two to talk to one another.

While you're more likely to see a six- or twelve-line phone in most large businesses, two-line phones still have their place. Whichever two-line phone you settle on, if you have a chatty household, the extra line capability will leave you wondering how you ever survived with just one. For more on telephones and two-line phones, dial up some of the links on the following page.

Related HowStuffWorks Articles

More Great Links

Sources

  • Crist, Sean. "Doing your own telephone wiring." (May 28, 2008) http://www.ling.upenn.edu/~kurisuto/phone_wiring.html
  • Donovan, Dave. "How to Hook Up a Second Phone Line." Do It Yourself.com. 2007. (May 28, 2008) http://www.doityourself.com/stry/addphoneline
  • German, Kent. "Two numbers on the same cell phone." CNET Reviews. Aug. 8, 2006. (May 28, 2008) http://reviews.cnet.com/4520-11282_7-6625917-1.html
  • Home Tech Solutions. "How Do I…Connect a Second Phone Line." 2008. (May 28, 2008) http://www.hometech.com/learn/hdi11.html
  • McAlevey et al. "Multifunction two line telephone system." United States Patent. May 12, 1986.http://patft.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=4691345.PN.&OS=PN/4691345&RS=PN/4691345
  • Michal, Keith A. "Adding a Second Line to Your Existing Phone Jacks." Phone-Man's Home Wiring Advice Page. 2006. (May 28, 2008)http://www.homephonewiring.com/add-line2.html
  • Recht, Thomas Stuart, et al. "Method of and apparatus for controlling operation of a multi-line telephone." Oct. 20, 1995. http://patft1.uspto.gov/netacgi/nph-Parser?Sect1=PTO1&Sect2=HITOFF&d=PALL&p=1&u=%2Fnetahtml%2FPTO%2Fsrchnum.htm&r=1&f=G&l=50&s1=5841851.PN.&OS=PN/5841851&RS=PN/5841851