Component values for a telephone line interface circuit

[Phaseseeker]

I'm working on an interface circuit for an old payphone I've been reverse engineering; the thing uses two phone lines, one which is an absolutely standard PTSN line used for voice, and a secondary line used for data communications.
This second line works with what they call "telegraphic" signalling; basically they're sending "packets" which consist of a long "start" pulse, then n short pulses where n corresponds to the digit being sent (think pulse dialling) and then a long "stop pulse". To achieve two-way data comms, for data going in the Central Office -> Phone direction is sent by inverting the phone line polarity, whereas the pulses going in the Phone -> CO direction are sent by the phone changing the load on the line (and thus the line voltage); there's an example of this in the attached pictures.
The line interface circuit on the phone side is quite trivial (again, see 2nd attached picture).
My problem is on the other side, where I'm trying to simulate the CO side of things; looking through patents, I've found a schematic of an actual line interface card used by the phone company (relevant part redrawn in the third picture), but the scan is poor and it's missing all the values for the components.
I understand the basic idea of what they're trying to achieve with this circuit, but I'm confused enough by the all the Zeners that I can't figure out how they would've picked the values for those.
I was also thinking of replacing the whole thing with a simple H-bridge + a high enough series resistor on the line to detect the change in current being drawn, but I'm not sure there's any subtlety in their circuit I'm missing (since the comms line also needs to provide power to the phone)...
Thanks in advance for any idea.
PS: In the line card schematic, keep in mind "earth" here is positive i.e. connected to 48V battery's positive terminal. That sent my brain in a loop for a while