It might have something to do with the lack of sidetone[1] (where you can hear your own voice through the earpiece), which had the effect of you naturally lower

It might have something to do with the lack of sidetone[1] (where you can hear your own voice through the earpiece), which had the effect of you naturally lowering your voice. Landlines alway had sidetone, but most cell phones don't. [1]: https://en.m.wikipedia.org/wiki/Sidetone https://en.m.wikipedia.org/wiki/Sidetone