Contemporary switches of all types are digital in nature, and so is a lot of terminal equipment. Most transmission facilities also are digital, with the notable exception of copper local loops serving residential and small business applications. Each sample is coded into an eight-bit digital value. The decoded signal, now in analog form, is only an approximation of the original analog signal, but it's thoroughly understandable to the human ear. It's not quite that simple, of course.
Timing is critical.
The network must be in a position to accept, switch, transport, and deliver every voice byte precisely every 125ms. Taken together, this approach ensures that, once the call is set up, the associated bandwidth is committed for the entire duration of a circuit-switched call, absolutely and without question. Voice speaks to circuit-switches, not frame switches and routers. Voice speaks to channelized, not unchannelized, circuits. Voice speaks to committed bandwidth from call setup to call teardown, not to bandwidth that's available whenever it happens to be available.