This symptom can occur on Macs that have some kinds of internal V.92 modems, specifically a V.92 "Dash" modem with a USB interface. If you don't know what kind of internal modem your computer has, use Apple System Profiler to find out. Click "Modems" in the Hardware section. If Modem Model includes the word "Dash," the Interface Type is USB, and you're using the Apple Internal 56K Modem (V.92) modem script, this document may apply to you.
With this configuration, if "Notify me of incoming calls while connected to the Internet" is selected, specific Caller ID details may not be displayed if someone calls while you're connected to the Internet; that is, the caller's phone number and name may not appear onscreen. Some telephone company equipment, or "switches," that your phone lines may connect to can lead to this situation.
If you have subscribed to caller ID and call waiting ID, you will see the phone number and name, if available, of the person who is calling you.
This document will be updated as more information becomes available.