Step 4: Modem to modem cabling.
How to connect two modems directly together. You can either do it with a simple battery or use a wallwart to supply power. All the parts should be readily available from most real electronic stores.
Note: if you are unsure about the connections, then get a professional to help. I will not be responsible for any issues.
Note that the resistor value depends on the actual voltage used. For 24V about 1K Ohms max will give at most 24mA (12V @ 500 Ohms, 9V @ 380 Ohms); the resistance of the modem circuit will reduce this slightly (you may need to reduce the resistor value, but if it works with the values mentioned, leave it at that; I'm using 380 Ohms with a voltage input of 14V). The telephone company guarantees about 20mA minimum in an actual phone line, and we want to be about the same minimum. Note also that the battery shown can be replaced by a "wall wart" power supply; most of these are un-regulated (my "9V @ 130mA" plug in DC supply gives about 14V on this circuit when connected to a telepone for testing) and consequently will need an electrolytic capacitor of about 2200 uF across the power supply + and - terminals to reduce the "ripple" voltage (i.e., AC "noise"); be sure to match the polarity of the Electrolytic to the polarity of the power supply.