Slew rate X telecoms

Audio quality in standard telecom cct's is limited by the CTITT codec,
so I would assume that there is a rule of thumb slew rate for 0dB
signals to get "irrelevant" 2nd harmonic distortion.

So is it possible to have a constant for any signal level? i.e. a
constant you can multiply by pk-pk signal to get the required slew
rate?

I get as far as saying dv/dt=cos(pi)=-1, i.e. theoretically vertical,
how do I translate slew rate limit into THD? And what would be an
acceptable value of THD for telecoms?

I suppose to find the THD for a given slew I would need to integrate
between limits at the point were dv/dt of a sin=slew rate of the
cct......but getting from that concept to the constant I am looking for
is way beyond me ;-)