Hello here, i am new to Vogons.
I have made this POTS emulator to be able to plug two modems and obtain dial up, this works by utilizing Windows RAS and the circuit i have made, tested it on winmodems and full modems and appears to work fine on both.
Most of the modems don't mind much the voltage on lines and ring, for ringer voltage the usual its 60 to 110VAC, but in my own testing my 22V transformer works just fine even on actual landline phones, with help of kotelstudios it appears a normal 12VAC transformer works just as fine for this circuit.
I chosen an 100Ω resistor since my Vaio W20 modem had issues with a 500Ω (wouldn't send any sound down the line and would send distorted sound to the speakers).
To use this circuit you must disable the option on the client "Wait for dialtone" if its greyed out you can put on the command line option "ATX1" and it should ignore dialtone.

To use this circuit you must have enabled RAS Dial-Up service on the server side (Preferably Windows XP, i had issues with 2k3 and Vista RAS not letting 9x clients login)
On the client side you must input any number, as long you have a way to tell the modem to ignore dialtone, when you click connect it will dial, then you press the button for half a second, let off for half a second, press it again and the server side modem should answer and connection should happen.

The maximum speed you will see on this kind of circuit its 33.6K since 52/56K couldn't survive begin converted from Analog to Digital

In case you only want to send files over Hyperterminal or anything similar you can complety skip the ringer circuit (7805 and relays)

To test the circuit without using the ringer circuit you can open Hyperterminal on both machines, connect both modems on COM, one machine put "ATX1" and the other "ATD1"

I have tested this on various modems
-Sony Vaio PCV-W20 CNR Modem
-Sony Vaio PCV-RX84M CNR Modem (Agere)
-Conexant CX11256 (PCI Modem)
-Intel MD5661 (PCMCIA Modem)
-Compaq Presario 425 2400 baud Modem (Rockwell)
-Thinkpad T400 Modem (Conexant?)

WARNING: I am not responsible for any damage done to your equipment.