This circuit uses MM74HC942, a single chip low-speed modem which is compatible with Bell 103 standard. This Bell 103 modem circuit is run at 300 baud in full duplex (bidirectional). The frequency and sine wave synthesizer inside this modem chip generate phase-coherent FSK modulation for the best bit accuracy. Here is the schematic diagram of the modem circuit:

bell 103 fsk modem circuit circuit schematic

This single chip low speed modem is suitable for remote data collection, radio telemetry, remote-process control, stand-alone modem, credit verification, tone signaling system, or point-of-sale terminal. The tone allocation of  MM74HC942, as any Bell 103 compatible modems is shown in Table I.  The modem on the end of the line which initiates the call is called the originate modem. The other modem is the answer modem. [Circuit’s schematic diagram source: National Semiconductor Application Notes]