The US telephone system has been digital except for the "last mile" since about the 80s. Speech is encoded as 8-bit samples at 8KHz, giving a Nyquist frequency of 4KHz, and using either µ-law[1] (US) or A-law (Europe) compression - you basically sample at 12-14 bits or so, take the logarithm of that value, truncate it to 8 bits and send it, and reverse the process at the other end.
If you take 24 of those 64Kbit/s channels and multiple them you get a T1[2]; 28 T1s were multiplexed into a T3. Basic ISDN multiplexed two 64KB channels and a 16KB signaling channel on a single wire, but it got very little use in the US.
The signals from those modems we were using in the 90s were getting digitized a few miles down the road, and at the ISP end the modem banks often connected directly with the digital phone system via a T1 line. (In fact "56K" modems relied on having a digital connection on the other end; the fastest modem speed with analog on both ends was I believe 33.6 with a V.34+ modem)
If you take 24 of those 64Kbit/s channels and multiple them you get a T1[2]; 28 T1s were multiplexed into a T3. Basic ISDN multiplexed two 64KB channels and a 16KB signaling channel on a single wire, but it got very little use in the US.
The signals from those modems we were using in the 90s were getting digitized a few miles down the road, and at the ISP end the modem banks often connected directly with the digital phone system via a T1 line. (In fact "56K" modems relied on having a digital connection on the other end; the fastest modem speed with analog on both ends was I believe 33.6 with a V.34+ modem)
[1] https://en.wikipedia.org/wiki/Μ-law_algorithm [2] https://en.wikipedia.org/wiki/T-carrier