My understanding is that plain old telephone service passes frequencies between 300Hz and 3400Hz. How is it that a V.34 modem in the late 90s would be able to achieve data rates of 33,600bits/s with this amount of signal bandwidth? That's nearly 11 bits/hz which is a significantly higher spectral efficiency than many modern technologies (e.g. DOCSIS 3.1, 802.11AC, 4G LTE, etc).
This article quotes the SNR of a phone line at 45 dB.
Combine this with a one sided bandwidth of about 3 kHz and you get a maximum channel capacity of about 45 kb/s.
45dB SNR is a perfectly reasonable assumption for a phone line, it's not great but certainly workable for speech. It corresponds roughly to the noise floor of a 7 bit A/D converter, so it's really not that outlandish.
Phone lines presumably have higher SNR than your typical power/interference limited radio link. Plus, wired links tends to have less reflections and be more stable, thus the fundamental limits bandwidth and SNR in a Shannon sense was probably easier to achieve than todays variable and reflective radio channels (requiring modern tech).