Yes these modems were almost-ISDN (minus the razor fast call setup). And required a full digital backend to work. They could only do 56k6 in one direction, to the user too. But they were made for internet access so that didn't really matter.
What I loved the most about ISDN was the quick call setup. Took like 1 second max and you had a 64kbit channel. 56k modems went through a dialling phase, a connection phase, endless handshaking...
It was cool having a fast downstream but the slow upstream over finicky dial-up was a pain in the ass. If the dial-up dropped the in progress downloads all died because no ACKs could be sent. Gaming was no better than plain dial-up since your upstream had the same shitty latency as plain dial-up.