Service providers in the wholesale space often sell their product based on per-minute fees as well, so it is easy to see why the working hypothesis is that voice actually is a commodity.
Despite all that, the way people use voice communications is anything but "commoditized," in the sense that one application is a fully functional substitute for another.
People who use landlines also use mobile and IP-based communications as well. People who use IP communications also use mobile and fixed calling. Likewise, mobile users avail themselves of IP communications and fixed services as well.
Beyond that, people tend to use each of the applications at different times, at different places, with different applications and different devices, to talk to different people, for different reasons.
Not enough attention typically is paid to the ways all those use cases can be differentiated in marketing. Usage already is differentiated in fact.