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to answer that one is easy… if I may
Digital technology that replaced central office relay switching works the same way when capturing rotary dial pulses generating make and break loop current at 45-65 % percent break. Later came touch tones on the basic service… to charge you more money 🙂 while saving all the maintenance $$$ on the clink-clank relays technology.
Digital ” line cards” simply capture the on-off current pulses on the phone loop as a stored digit and converts it to ” signalling/routing) information” to be processed further down the system . The accumulated dialed numbers ( 1 to 0) interspaced by ” inter digit timing” are accumulated to form a destination # according to the Number Plan Area +Exchange NXX & xxxx format.
Computers then first define the call as local or LD & establish a point to point connection and pass on network routing info to both calling and called parties ( dial tone, ring tones busy tones, ” no such number tones, netwrok messages, called ID) etc..)
Upon getting a signalling reply (off-hook loop current ) from the destination the voice component is established between parties. All this is done in an instant
sound quality wise… old networks carried 300-4000 hz bandwith for human ears… today everthing is sampled, encoded, reassembled using complex algorithms to recreate sounds … no wonder ” bits” of it are lost on the way 🙂
Concept wise … nothing new under the sun. remember when phone serbvice was $18/month 🙂
One problem…with rotary… not so good to fetch /control voice mail unles you link it to a separate touch tone pad 🙂
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