I am puzzling over this schematic, the datasheet just says to follow their recommendations exactly, and doesn't give explanation of the IC actions or circuit functions.
The lower half is easy to understand, it deals with incoming rings, and splitting and directing the signal with the bridge.
I assume that R1 and c4 deal with the received signal.
The upper right part has me perplexed though. I assume between Q4 and Q3, one is the offhook signal, and the other the output. R2, Q5, and R11 appear like they could be the output gain, although they tie into Q3 as well??
I really want to up the gain on this circuit, our existing church sound system install has a circuit designed around this schematic, and I find it is not fully saturating the phone system adc, our volume is too low. The previous system we had based on a transformer had much better volume.
I have punched this into LTspice, its a little unclear at this point. Voltage on DCT gets the best response, as partially predicted by an answer below. I have use voltage values on pins based on SI AN94. A sine wave on DCT is the only that outputs a reasonable value. Not sure if I have telephone line modeled correctly, but ma shown seems reasonable. LTspice model