This has produced some interesting comments, here's my pennuth:
My voice varies in tone pitch and usually wording, as do most (except monotone half bro) depending on both the topic and the person(s), not whether or not I'm on the phone. I know this is so as I do video conference calls in which nobody has a phone or mic, and there may be up to 6 people in each of 3 or 4 places and so 3 (or 4) mics are live, one in each place. Other members also sound the same ''live'' or alive in coffee/lunch break, again depending on both the topic and the person(s).
I used to speak deliberately clearly in the 70s for secretaries who would be typing from the Dictaphone. I now speak similarly, and more slowly, with new lower level English students.
I used to enjoy ''doing accents'', especially with another person and of course to do many, the voice has to be pitched in a different range with different tones - the basic difference in accents (and obviously languages) is firstly the vowel sounds....
trapio