The Ttranszorb is a good idea. But what is it? What is its specs?
Other Qs are:
You gave no value for the "R?" in the input wire. it should be the
same as the other R1, 100k, so that the line stays balanced.
Why is the tranzorb _after_ the 2 resistors and bridge? I would think
it should be before, to protect those components.
My thought is that by the time the fault voltage gets high enough to
cause the tranzorb to do its job, it has already caused damage to the
LM393 and other components.
I would include two clamping diodes after R5, to the + amd - power
supply. It's also a good idea to put two diodes anti-parallel across
pins 2 and 3 of the LM393 to prevent overvoltage. These are some of
the ways we used to protect the line driver boxes we used all over
campus. Believe me, when a lightning storm comes by, you will need as
much protection as you can get to keep your phone line-connected