I wonder if the phone lines are still compatible with data modems nowadays, or if they're all Voice over IP.
If they're Voice (and other noises) over IP, then the modem would be encoding IP packets to tones, and as soon as it reaches the phone provider's hardware the tones will be digitized to be transmitted over IP packets again...
There are some other standards, but they have to be supported on both sides of the line, which is not a given, especially not between different TELCOs/ISPs.
Which makes trying to use FAX over it really 'interesting'.
When we moved into a newer subdivision just before 2000 we discovered our modems couldn’t get over about 19.2 (IIRC). We did a lot of troubleshooting but in the end found the culprit was how the phone lines were setup in the complex.
I think the lines were basically like VOIP from the box at the street (or somewhere nearby). They did it to pack more lines in, but it limited the bandwidth available.
You had to pony up for an ISDN line if you wanted to go faster until cable internet was available a few years later.
If they're Voice (and other noises) over IP, then the modem would be encoding IP packets to tones, and as soon as it reaches the phone provider's hardware the tones will be digitized to be transmitted over IP packets again...