First, which USB modem do you have? Second, Phone Amego includes a "Phone Amego / Debug Log" that allows you to see the exchange between your modem and computer. You can open this log window under the "More" tab, or by pressing Control while you select Phone Amego from the menu bar item. The log should give us a clue what Phone Amego got from your phone and whether it should have displayed some caller ID text. Are you using Phone Amego's built-in "Call Status Display", or only Growl? Which computer failed to display the caller ID as expected.
Please Email me directly with more detailed debugging information. The forum is best for questions and answers that are likely to be of interest to other readers.