We used to use the old AT&T Dataphones (401E?). Amazingly few active components--LC tuned circuits mostly and a very few transistors.
I think there was a general mistrust by AT&T of transistors at the time--the standard Touch-Tone dials were single-transistor affairs (to generate two tones at once, no less).
Anyone here own one of the do-it-yourself modems from Popular Electronics--the "Pennywhistle" modem?
I connected my MITS Altair to the system at work with the guts from a TI Silent 700 terminal. A few years later, I moved up to a Racal-Vadic "triple" modem--300/1200 and (ISTR) 1800 Vadic protocol.