It might not be a problem for end-to-end transmission. However, for stations tapping in the middle of the line. The main line now become branches, which are quite significant (between 1/8 to 1/16 wavelength).
I am sure both ends are open (unterminated). If the OP idea is to make use of unconnected phone lines.
Not a simple signal drive by I2C (open wired-or) for instance.
Higher voltage is needed because of lower impendance at higher frequency.
Different bus configurations, not cable differences.