I have a hardware device I need a server to dial into at regular intervals. The problem is I no longer have a POTS line or a modem in any of development computers, and all my production servers are virtual. I went out and got a usb modem for development and will plug it into a broadvoice VOIP line, but this seems quite kludgey. I've googled around and have found "virtual modems" that simulate serial connections or "at based" modems over IP, but so far nothing supports SIP or any kind of VOIP.

Is there a way to connect a virtual modem to a real one?


Update Putting a computer near the device isn't a good option as its in a public basement, (though the ALIX boards might work) and even then I'd have to dial into it. I also can't replace the modem with a serial device or an ip device as the only connection to location is the phone line and I don't know enough about the device to modify it. This is for some old smart meter equipment.

  • Your problem seems to be more of a physical one than software. Your options will depend upon the environment. If the smart meter is in the same building and your server is close to the punchdown panel, you MIGHT be able to hardwire a connection and use RS-485 adapters, but these are difficult to get working over UTP cable. If you have no POTS lines in your server room and no structured network cabling in the basement, you are going to have to fix the problem on one end or the other; get a true POTS line in the computer room or extend your network to the basement (cabled or wifi). – pbrooks100 Jan 7 '10 at 14:38
  • You need the client & server device to support the T.38 standard – Nick Kavadias Jan 27 '10 at 3:47

To save your sanity, don't.

Modems (and fax) over VoIP are extremely sensitive to timing, and usually break over VoIP, it might even seem to work at first, but it's certain to break.

Adding virtual machines just makes the problem worse.

There are "short haul modems" around which can emulate a phone line.

Another option is just to attach the two modems back to back:


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  • I'd have to install a computer in the basement were the device is. I'm better off installing a computer where a phone line is =p – reconbot Jan 6 '10 at 6:50
  • Or you could just run a cable – LapTop006 Jan 6 '10 at 6:52
  • I think a small computer and a short haul modem might be the best bet. The computer can then get on the net via dial up and provide the data. – reconbot Jan 7 '10 at 20:35

I have a similar need, and I've been searching for hours and hours for a solution. The closest I've found is a free but not very well-written program called "TimeFax" (http://sourceforge.net/projects/timefax/), which installs a virtual COM port and connects it to an H.323 receiver. You can make Windows install a legacy 14k modem on that COM port, and theoretically the program will take the COM port data and transform it to H.323 and pipe it to whichever receiver you specify. Then all you need is a VoIP H.323 termination, and you might be good.

Note that to get TimeFax working on Windows Vista or 7, you have to disable desktop composition in the compatibility settings. You'll also have to manually download the 2 dll files that it is missing, which it will complain about when you first try to run it. A quick Google search finds them easily though.

You'll still have issues with VoIP compression, but if you lower the speed, you might be able to achieve something that works. A few years ago I routed a dial-up modem through a Vonage connection, and it worked surprisingly well if I kept it at slow speeds (like 14k), so I know that dial-up over VoIP is possible, I just haven't quite got a completely software solution working.

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We use Cisco ATA 186's to on all of our faxes and as a backup in case our WAN goes down and we need to get into our networking gear at the Datacenter ... 1900 miles away. They've served us very well


In that case of purely software devices ... they don't exist. I tried looking high and low. Including trying to hack something together with asterisk ( with the help of an asterisk hacker i work with ). I was looking for a situation where i could dial into the modem on our network gear anywhere even when all I had was my Cell card.

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  • I'm looking for more of a software solution, I don't have any physical lines anywhere besides the device, and I don't have hardware access to my production servers. On the other hand I do have vo-ip accounts. – reconbot Jan 6 '10 at 6:52

I'm not an expert, but as an aside that I find a bit too long for a comment:

Even "true" POT lines use multiplexing to handle multiple calls over a single cable. This uses conversion of analog signals (the tones generated by the modem) into digital ones, and back. The digital capacity is limited by design, and thus the analog bandwidth is limited too. VOIP uses its own compression as well. Hence mixing the two, if possible at all, might only work with low speeds.

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  • I know there are several different codecs and the ones optimized for voice are not good for data. I don't know much else though. – reconbot Jan 6 '10 at 20:40

I can't say that this would work but it seems to be at least close:


While I've not used DataRemote software I have used their hardware and dealt directly with their very accessible tech support for more than 3 years now. I'd give an A+ all around for my own personal DataRemote experiences.

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When this question was asked, there was no existing solution, but Vocal Technologies has a SAMS (SIP Analog Modem Server) which is what the OP was looking for. It is a purely software modem server that can connect to actual Physical Modems connected to the PSTN, from a Cloud server connected only via SIP/G.711.

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You might be able to use a serial device server such as the Lantronix UDS 1100.

You load com port redirector software on the host computer and connect the device server to your remote device in place of a modem.

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(source: lantronix.com)

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