I seem to recall from back in the dark ages . . .
. . modems over dedicated lines were very different from dial-up modems and not interchangeable one to the other (except a few models that could handle either through separate connections) - and yes, the telco dedicated line did provide voltages.
I also recall "CO emulators" were sold so demos at shows could talk to each other. CO emulators provided telephone line voltage, dial tone and ring voltage so dial-up modems could talk to each other.
Now this is very hazy, but I think there were modems designed to talk directly to other modems, mainly for in-plant industrial communications.
