If you already have some form of broadband internet (cable, dsl, etc) then the modem is pretty much useless unless:
- You are actually dialing an ISP or a remote computer.
- You are manually "calling out" and "picking up" on 2 machines whose modems are directly connected by a phone cord.
If you have the machines physically in the same room, house, etc, then I'd go with Bunsen's advice and "skip the modem altogether." The modem will severely limit you in terms of transfer speeds. (up to 56kbps, depending on the model and age of modem.) It is far easier to use a serial or Ethernet network.
Using the serial port on the Classic you'll be able to transfer up to 230.4kbps. With Ethernet on the PowerBook G3 and iMac, you'll be able to achieve up to 10/100Mbps between them. (depending on the model of the PowerBook.)
I noticed you have a
similar topic over here. In there you mentioned your long term goal is to get internet access on the Classic. Have you looked at any of the networking guides that
Bunsen recommended yet? Out of the list, I'd highly recommend Mk. 558's
Classic Mac Networking Guide 3.0. As it is one of the most comprehensive guides out there, and includes multiple methods to networking Macs. It also give ideas as what you can do with the Classic once you have it connected. (IRC client, Telnet, SSH, etc.)