Modem Hangs Up Before ISP Answers

When dialing ISP modem dials, but then disconnects before the tones from ISP begin. Listening from handset I can hear the tones begin just a second or 2 after modem has disconnected. Tried both a hardware ISA modem and a software PCI from 2 different locations (just in case of lines). Exactly the same result on all accounts. Probably going to reinstall DUN.