I've found that many modems are Winmodems. These are not
hardware-controlled, but software-controlled and are designed to work with
Windows (better said, let Windows do the work for it).
Have you checked to see what type the USR 5633 is?
I have a USR 5610b that I bought ($11 on ebay) recently to replace the
Winmodem I had been using. I've managed to get it to dial and connect, but
not recieve once it connects.