Thanks for your response, John. The phone repairman showed me the readings on his meter checking the phone line: It read within spec when the phone was disconnected from the computer but too high when running through the modem, which seems to indicate a problem with the modem, not my ISP.
The modem is a Creative Modem Blaster V-90 PCI DI5655. I'll check the web to see if there is any info.