Siemens TTY-TTY cable

I know I can order a Siemens cable for TTY to TTY communication, but it is very expensive.
I found a wiring diagram for this cable, but there seem to be
zenerdiodes in it. Are they realy necessary, and if yes, what specifications do they have?
Has anyone experience in making this cable by himself?
It's for communication between S5 and OP77B.

Hi
I dont have zenerdiodes in my cable, so as my collegues. We never had any problems with those cables.