# Phones: The 555 Exchange Is In-Use? **URL:** \ **Category:** Factual Questions\ **Created:** [May 13, 2004, 5:15pm UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973 "2004-05-13T17:15:55Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![Tamex](https://avatars.discourse-cdn.com/v4/letter/t/ccd318/32.png) [@Tamex](https://boards.straightdope.com/u/Tamex)\ **Post date:** [May 14, 2004, 1:03am UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973/21 "2004-05-14T01:03:55Z")
> [@TellMeI'mNotCrazy](#): > > Not sure where you come from but true enough, states can have several area codes, not aware of any cities that have multiple area codes though; as far as I’m aware, they do their best to keep it as simple as possible, and saying, “Well if they live in the western half of Danbury, you have to dial 203, but if they live in the eastern half of Danbury, you have to dial 860”, would be downright confusing. So I’m curious as to where you are - I’m assuming not in the states - and whether you’re confusing area code for something along the lines of a city code. Some areas have overlays. The people who already have numbers have one area code, but people who get new numbers get them in the new area code. The advantage is that no one has to change their number. The disadvantage is that seven-digit dialing no longer works at all in those areas, and your next-door neighbor could have a different area code from you. There are also other cities that are so large that they have multiple area codes, like New York and Chicago. There are also still a handful of states that have a single area code. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [May 14, 2004, 1:05am UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973/22 "2004-05-14T01:05:35Z")
> [@TellMeI'mNotCrazy](#): > > Not sure where you come from but true enough, states can have several area codes, not aware of any cities that have multiple area codes though; as far as I’m aware, they do their best to keep it as simple as possible, and saying, “Well if they live in the western half of Danbury, you have to dial 203, but if they live in the eastern half of Danbury, you have to dial 860”, would be downright confusing. So I’m curious as to where you are - I’m assuming not in the states - and whether you’re confusing area code for something along the lines of a city code. My city has three area codes. When the 2nd was added, they split the city geographically (but not in a logical way), which ticked people off big time since it was unnecessary for half the city to change area code. When the added the 3rd, they made it an overlay. (New numbers get the new area code.) I.e, what they should have done the time before. Now a _large_ city … ---
**Author:** ![mks57](https://avatars.discourse-cdn.com/v4/letter/m/b9bd4f/32.png) [@mks57](https://boards.straightdope.com/u/mks57)\ **Post date:** [May 14, 2004, 1:26am UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973/23 "2004-05-14T01:26:24Z")
> [@TellMeI'mNotCrazy](#): > > That just furthers my question… How does one spoof a caller id number? Seems like that would take quite a bit of work - especially to show up as 555, which as you say is pretty obvious. Having anonymous or out of area usually raises flags too so why not just let the true number show through - it would at least be less suspicious than 555-0216? Telemarketing lines usually don’t allow incoming calls anyway. But in any event - is spoofing definitely possible then? And if yes, then how? (I have no interest in doing it, of course, but it seems to me this is a pretty shady thing to be able to do.) Caller-ID works by having the central office send a short burst of modem tones containing the Caller-ID data after the first ring. I believe it uses Bell 202 (1200 bps FSK) modem tones. Your Caller-ID unit has a receive-only modem that decodes the modem tones and displays the data. Since this is in-band signaling, meaning that the data is transmitted over the same channel as the audio, the calling party may be able to transmit “fake” Caller-ID data by transmitting a burst of modem tones after the central office has finished sending the real Caller-ID data. ---
**Author:** ![Q.E.D](https://avatars.discourse-cdn.com/v4/letter/q/51bf81/32.png) [@Q.E.D](https://boards.straightdope.com/u/Q.E.D)\ **Post date:** [May 14, 2004, 6:56am UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973/24 "2004-05-14T06:56:22Z")
> [@mks57](#): > > …the calling party may be able to transmit “fake” Caller-ID data by transmitting a burst of modem tones after the central office has finished sending the real Caller-ID data. That won’t work. The CO doesn’t send any audio from the calling party’s circuit to the called party until the connection is completed by the called party picking up the phone. But, once the CID unit detects the off-hook signal, it no longer “listens” for modem tones. ---
**Author:** ![muldoonthief](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/muldoonthief/32/3188_2.png) [@muldoonthief](https://boards.straightdope.com/u/muldoonthief)\ **Post date:** [May 14, 2004, 5:45pm UTC](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973/25 "2004-05-14T17:45:30Z")
> [@DesertDog](#): > > A long, long time ago when area codes were first being set up, all of them had a 0 or a 1 in the middle. If the area code was for the entire state, it had a zero (Like 602 for Arizona) and if it was only part of a state, it got a one (Like 215 for Philadelphia). That neat little rule was the first to bite the dust as populations grew. 602 was reduced to the Phoenix metropolitan area with the rest of the state getting reassigned. It was further reduced to Phoenix and west valley, then Phoenix-proper, then central Phoenix. Meanwhile, even the only zero or one in the middle rule was dropped, and area codes can be pretty well anything. Except for the toll-free 888s, I haven’t noticed any triple digits, though. > > DD Also, area codes were assigned to make certain numbers easier to dial on old style rotary phones. With a rotary phone, the amount of time it takes to dial is proportional to the sum of the individual digits, with 0 being 10. So “212” is much quicker to dial than “909”, so the area codes were somewhat assigned according to how many calls they expected an area to receive. That’s why big cities got the 21x numbers - New York 212, LA 213, Dallas 214 etc. [Previous page](https://boards.straightdope.com/t/phones-the-555-exchange-is-in-use/244973.md?page=1)