A quite effective way to produce the FSK waveform to stuff onto the phone line is to simply make square waves out of your processor, similar to the way I described before. Then couple the signal via a small transformer onto the line. The primary side of the transformer can have a capacitor tied across its primary and a series resistor from the port pin to one side of the transformer. You could try connecting it as shown below and use a CMOS buffer gate to drive the transformer with equal drive in the high and low states. A TinyLogic part such as a 7SZ125 could be used as the driver.

It may be even possible to omit the hook switch / relay contact that connects the circuit to the phone line and and just tri-state the 7SZ125 chip when the FSK is not desired on the phone line.

The circuit will put a signal that is surprisingly sinusiod onto the line provided the appropriate transformer and primary capacitor are selected. Keep in mind also that the CLI detector chip on the other end of the line does not really need to see a sine wave anyway. Usually an FSK decoder amplifies the signal from the line with a high gain circuit that clips its output to the supply rails and then uses a zero crossing detector (comparator with hystersis) to make an internal square wave out of the FSK signal. The FSK signal is then decoded by measuring the pulses phase and frequency.

Michael Karas