Yes, you're right, I totally missed them. Those look like USR 'Courier' modems but the resolution is really crappy so hard to be sure and it looks like there are multiple types. There might still be modems in the boxes themselves as well. It doesn't look like more than two modems per box if there isn't.

yes, most of them look like USRobotics Courier modems. Note that not all the machines have one, and some have two.

Assuming that the parent commenter is right and that they are using internal line cards, I wonder if the external modems were being added to support higher speeds.

However, the fact that we can see at least 2 (but I think four) 66 blocks means they had 50 to 100 phone lines for the machines visible, which would make sense that the external modems are the primary connection and no internal modems are being used, based on the number of modems visible and the fact that each 66 block can handle 25 lines.