the pitch of our voice cause the vibration of magnet in the microphone, thus causing generation of different voltages of electrical signal.
this is not exactly true. the voice doesn't have the pitch. the pitch is a concept from a frequency domain. when you speak you change the air pressure over time. these changes travel in air with speed of sound, they hit the mic. mic records these air pressure changes as electrical signals of different amplitude, like show in the picture on top, where i recorded my saying "hello".
you can run spectral analysis on the entire recording to convert the time domain signal recording into the frequency domain, you'll get the periodogram in the picture in the middle. as you see there is not a pitch there, but rather a continuum of pitches: all frequencies are there, some with higher intensity than others.
you could also argue that the pitch changes with time, and get a spectrogram on the picture at the bottom. it's a sequence of periodograms over time, showing that the frequencies change over time.
basically, all that i'm saying is that the concept of a pitch of the voice is rather complicated. when we speak over the internet phone, the mic records what's on the first picture, and the speaker on the other end tries to reproduce it. in the middle of this all many things may happen, such as compression and encryption. you can lookup the details in documentation for SIP