I think something regulatory changed about 1995, such that there were instantly tons of ISP startups, and I think that 33.6k modems were available about then.
I had a 14.4K Zoom modem, but I'm pretty sure the ISP I worked for around '95-'96 was buying lots of US Robotics 33.6 modems.
I agree that 56k came a bit later, and didn't necessarily work on any particular phone line.