3dnow wrote on 2025-10-31, 19:12:

I can't wait to see that fully assembled!

Unfortunately this will take some time. Too many important tasks right now, but I think around Christmas I'll find some time tinkering with it and do a follow up.

As someone who worked year "in the business", designing both phones and
phone systems - I can tell you that interfacing to an analog phone is easy.
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In a recent project involving a DTMF receiver, and not having any actual DTMF
keypads anymore, I built this little interface to obtain the audio from an
actual analog phone as it dialed.

It consists of a phone jack, in series with a current limiting resistor and
a transformer - powered by 12v supply (as I was interested in extracting as
pure tones as possible, I did put a capacitor on the supply directly at the
interface).

The transformer is a typical 120->14v used in many supplies, tone frequencies
are close enough to its designed 60hz line frequency, and connected in reverse
it acts as a "step up". From this setup I can get fairly clean DTMF of several volts,
with a small pot to adjust it down to the levels I want for my test curcuit.
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Getting an analog modem to connect to the internet is quite a bit tricker,
for many modems you need to provide "dial tone" before it will even try to
dial, then you need to provide an audio-path to whatever you are trying to
connect to (most likely another modem set to answer - in this case you would
also have to generate ringing to get it to answer - If you have your own
software handling that end, depending on the modem you might be ablle to get
it to go off-hook in answer mode via a command).
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At the left of the pic. is a "TalkSwitch" - the smallest phone system I worked
on - as small as a desk phone, this little gen provides 8 extensions (which can
call eash other) as well as 4 C.O. line interfaces, and some of them even have
a VOIP interface. These are my go-to whenever I actually need to simulate a
working analog phone line.
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