What is the best wire (material, construction and gauge) for a 30 meter wire telephone?
You want very stiff taut wire.  Otherwise you will have to pull it VERY tight to pull it to the limits of its springiness so the sound won't get dampened by that spring action.   

Also preferably nonconductive.   Maybe something in the FRP/carbon fiber family.   

My vote is for fiberglass electrician's pulling tape.   Nonconductive, stiff and mad tensile strength.