I'm sure I found the same sources you just did. USR101 is surely not a ROM by this [0] schematic because the speaker audio goes directly to one of the pins on it - that doesn't seem right.
I guess enough signal makes it through T1 to let it work on dry lines. That sort of thing isn't unheard of [1]. I used to mess around with obsolete dial up modems as a kid and I could definitely get them to connect with just wires - sometimes. It didn't always work, and some modems would not work with other ones, etc.
The weird thing here is the 1670's second long pulses. Maybe it would do something else if connected to a real phone line? I'd surely try it myself but I only have a 1660 modem on hand.
I guess enough signal makes it through T1 to let it work on dry lines. That sort of thing isn't unheard of [1]. I used to mess around with obsolete dial up modems as a kid and I could definitely get them to connect with just wires - sometimes. It didn't always work, and some modems would not work with other ones, etc.
The weird thing here is the 1670's second long pulses. Maybe it would do something else if connected to a real phone line? I'd surely try it myself but I only have a 1660 modem on hand.
0 - https://www.zimmers.net/anonftp/pub/cbm/schematics/misc/1670...
1 - https://en.wikipedia.org/wiki/Sound-powered_telephone