I ended up running a 50ft phone cable to the telephone interface box outside, then connected that directly to the computer modem. It worked with the direct connection, so it's obviously an indoor line noise issue. That surprised me because I have a USR 56k V.Everything Courier modem that I've used in the past, and never had a problem connecting online. I guess the cheapo modem in their computer wasn't as good at dealing with the noise. Thanks for your help!