The IBM history "IBM's Early Computers" by Bashe et al talks about early transmission efforts in the 1950s using modulation over voice lines. I believe this had to be done over private lines only since it was their gear, not Bell's. IIRC, IBM didn't push too hard in this area since they didn't want to anger Bell which was a major IBM customer. IBM was experimenting with various things in the 1950s, using including Western Union telegraph lines.
Earlier, IBM also developed a radio teletype which presumably had to modulate and encode key characters.
Bell came out with the Dataset in the 1960s. I suspect Bell may have had experimental installations for customers and itself before that.
How much cooperation there was between Bell and IBM on data communications in the 1950s I don't know. Their respective histories don't seem to talk about it much.
IBM's Watson Jr says the company was a bit insular and missed a 1950s opportunity to jointly develop xerographic printers in cooperation with Haloid-Xerox.
As to other comments in this thread:
1) Dial up modems were available well before 1973, at least as earlier as 1968 as part of Teletype time sharing terminals. These were automatic in the sense they would answer a ringing phone line and connect up and disconnect and power down upon completition. They could remotely start/stop different tasks, like the paper tape punch and reader.One thing hasn't changed in all these years -- the high pitched answer tone we all hear and the responding tone. The subsequent handshaking may have evolved, but that original irrtating sound remains the same. The automatic teletypes had a speaker built in so one could hear it.
2) BBS: They were a lot like Usenet groups. The quality varied greatly from BBS to BBS and from poster to poster. Flame wars, which got way out of hand, were common. So were "bullies" and self-appointed "police". There was useful information as well plenty of garbage masquerading as useful information.I noted that there were very passionate techie advocates of BBSing as the "wave of the future". In truth, a very small percentage of the general population were involved in it.
Around that time commercial services were attempting to provide features that the Internet would later give. Prodigy was an early one, designed for easy home access. Compuserve was the big boy. (I was a Compuserve subscriber but hardly ever used it because anything of interest cost extra.)