How to Digitize Analog Voice Signal?

Two suggestions for you. Get a good book on digital signal processing, on that covers audio. Second, go onto any major silicon vendor's website and download data sheets and application notes on their CODEC ic's. There should be a wealth of info about the processing that goes on in the chip and you may find that it will be easier for you to choose one of these pre-developed solutions, since that is why they exist, to keep you from having to do it all discretely.

I agree. When I started my first electronics projects I started by trying (the hard way) to learn assembly code and reinvent the wheel when there were many pre-developed and debugged solutions out there. Many digital systems are off-the-shelf parts that just have the right software on them

Agree with both posts so far, and don't know anything about mobile telecomms., but, in PCM (wired comms.) step three is broken into two parts first: convert analogue to 13 or 14 bit digital then: convert 13 or 14 bit digital to 8 bit digital using a kind of piece wise (chords) logarithmic scale. Each (8KHz) sample always occupies an eight bit word (in both Tx and Rx directions). In mobile comms. I believe that more processing is done to compress the data sent over many samples.