After installing Mac OS 8.6 on an original iMac, the Modem control panel defaults to the Infrared port. Here are the steps for reconfiguring:
1. Open the Modem control panel.
2. Choose "Internal Modem" in the "Connect via:" pop-up menu.
3. Choose iMac Internal 56K in the Modem pop-up menu.
These settings will be saved in the Modem Preferences file.