There are ways of allowing fax machines to share the same line as telephones and telephone answering devices. Back in the BBS days, a lot of people had dedicated phone lines for their BBS. A caller need not have their own dedicated line to dial in to a BBS, but you basically needed your own line if you were the SysOp. Computers and teletypes can be acoustically coupled to send and receive data with other computers over the PSTN. Likewise, I am looking into an effective way to set up a telegraph, a teletype or two, and some modems.

There are two different types of PSTN calls:

  • Voice calls (a person calling a person, or leaving a message on a TAD)
  • Everything else. A computer calling a computer, a fax machine calling a fax machine, a TTY calling a TTY, etc. (calls that should automatically be answered).

It would be nice if I could connect them all to the same line, which works for outgoing calls but is tricky for incoming calls. If I receive a voice call, I want the phone to ring indefinitely. Anything else should be automatically answered. I don't think it is possible to do voice and data on the same line like that, so I would go to two lines - one for voice and one for data, both with Direct Inward Dial obviously. I could hook these up all to their own line, but that would just be a waste of money, since they probably won't ALL be in use at the same time.

Direct connect modems can automatically answer calls, but that assumes that they are on their own line. I know fax machines are able to detect fax noises and then send output to a TAD if it is not a fax machine calling. Is there some sort of "supermodem" that is able to filter and route calls appropriately?

For example, a modem that has a jack labeled "FAX", , a jack labeled "TELEGRAPH", and a jack labeled "DATA" (for computer modem, connected either to a teletype/teleprinter or a terminal) (and then no jack for voice, since that would be impossible). All of these calls should be answered immediately, which avoids the conflict with letting the phone ring if a person is calling but answering immediately if it is not (since there is no way to tell until the line is picked up, this is impossible).

I know computer modems emit tones (Carrier Tones) to let the other modem on the other end know a computer is there (this is how war dialing worked). I know fax machines have their own distinctive tone as well.

So, taking advantage of the four different data types having their own distinctive tone, is there (or could I design) a modem that will automatically pick up incoming calls, quickly listen for the type of tone (carrier tone, fax tone, telegraph) and then 'route' the call to the appropriate machine?

I believe telegraphs have always used dedicated circuits and were not used over the PSTN, so they don't have their own tones. Hypothetically, let's say a telegraph modem exists and it emits its own distinctive tone and would be connected to any machines in question.

And in this manner, I would be able to use any of these machines on the same line (not at the same time), or automatically have the correct machine receive incoming calls.

Basically, what's the question mark in the diagram below? If it doesn't exist, could it be designed?enter image description here *Note, telegraph SEND could also include key/telegraph RECIEVE could also include a sounder, etc. (a lightbulb even) **Telegraph modem would include a rotary dial since telegraph is not on a dedicated circuit and calls need to be routed - i.e. to other telegraphs on the PSTN - or simply allow a telephone to be connected so the modem can route and determine off or on hook status if the telegraph does not have an on/off switch. Telegraph modem might also convert telegraph "pulses" to noises of some frequency ideal to send over PSTN, and convert incoming noise back to pulses telegraph READ (sounder, printer) can understand.

**Teletype would have to be connected to an acoustic coupler, not a 56k modem.

***Also, an ANS/TAD port could be added for an answering machine, so that if no tones are sent from the calling party, the ?device automatically defaults to the answering machine so that if a person called the line to leave a verbal message, they can do so.

****The line going from the PBX to the ?device would be intended for 1-way communication, not 2-way. That means either the devices could be used to send out, or they could receive telegrams, data, faxes, messages (answering machine).

  • I'm not sure I follow you. For example, a fax call is a voice call. That was the whole point of a fax machine is that it uses a regular voice line. The same thing holds true for analog modems. You would frustrate a person calling because whatever answered the phone line would need to send obnoxious noises like fax or modem tones when it answered the call.
    – Ron Maupin
    Aug 2, 2017 at 13:29
  • @RonMaupin No, this line going from the PBX to the thing with the question mark would be for data only, i.e. voice. I suppose an answering machine could be hooked up so that if no tones are detected from the calling party, the answering machine will kick in. By voice call, I mean person talking to person. All of this would happen over an analog copper circuit though, yes. Aug 2, 2017 at 15:44
  • You forget that the device must send the tones to the caller, first. When you call a line with a modem, you will hear it send tones upon answer.
    – Ron Maupin
    Aug 2, 2017 at 15:45
  • @RonMaupin Yes, but computer modems for instance automatically pick up, unless you have an acoustic coupler. This circuit would be dedicated just for "messages" i.e. fax, telegrams, BBS, teletype, etc. and would not be intended for simultaneous 2-way conversation. This avoids the paradox of letting the line ring forever if it is a person wanting to talk and picking up right away if it is not. I would use other lines for voice. So, based on the tone of the calling party, would this ?device send back one of the three appropriate tones? (or default to answering machine if no tone). Aug 2, 2017 at 15:49
  • What you are missing is that the answering party must initiate the tones. The answering party has no way to know if the calling party is a person, fax machine, or modem. Some people do this with caller ID, but a voice call is a voice call. Unless the caller responds to the tone the answering party sends, the answering party has no way to determine if it is a data call.
    – Ron Maupin
    Aug 2, 2017 at 15:52

1 Answer 1


Apparently, you are overthinking this.

With a PBX you get as many PSTN lines as necessary for the total number of simultaneous PSTN calls that you may have. Usually, you get a DID range that is larger than the number PSTN of lines. You could have a single incoming PSTN line, but separate numbers in a DID range for each internal function. The PBX can then direct the call to the appropriate internal device based on the called number.

  • I know I could do this in a PBX without the ?device, although a) I don't want to pay for a really large DID range, and b) I wouldn't want to use every number in the DID range (i.e. maybe numbers that end with 0 or 5 are DID and others are private internal numbers). But not everyone has a PBX. I'm trying to figure out how someone with only 1 landline (average joe) could have all these devices set up, so that if someone wants to send them a fax, they can do so. If someone wants to send them a telegram, they can. Or leave a message, etc. Obviously they can't do more than 1 thing at the same time. Aug 2, 2017 at 16:20
  • To clarify, a way for all these devices to be accessible from outside without the need for 4+ individual landlines. I think that 2 might be required, again because of the person calling to talk or not paradox. But if we take voice calls out of the equation (except for leaving a message on a TAD), I need a ?solution that would allow these devices to all be used on one line, albeit not simultaneously. So, if I want to send Joe a telegram, I call his # and am connected with his telegraph. If I want to send him a fax, I call that same # and leave a fax. Make sense? Aug 2, 2017 at 16:23
  • I explained how it is done. The quantity of phone numbers does not need to be the same as the number of PSTN lines. You could have a single incoming PSTN line with four phone numbers, and each of those phone numbers could be switched by the PBX to a specific internal extension. That gives you separate numbers for each function without requiring a separate PSTN line for each function. This is done all the time. The PBX gets the called number from the telco and decides where to forward the call based on the called number.
    – Ron Maupin
    Aug 2, 2017 at 16:28
  • @InterLinked, you seem to be trying very had to make this into an X-Y problem. Your problem has had a solution since the invention of the PBX.
    – Ron Maupin
    Aug 2, 2017 at 16:30
  • For someone WITHOUT a PBX? They have no way of routing calls based on DID. And I don't think people will want to pay for the additional #s. Plus, that seems a waste of #s when this could be accomplished in a somewhat different way. I can't expect everyone to have a PBX and be paying for 5 DID numbers. +Are you saying that a modem stops modulating the line/emitting tones after a number is dialed. The last time I checked, modems are always emitting tones, even when not being used. Aug 2, 2017 at 16:30

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