Context: Debian Etch 4.0r0 with external 56k modem.
Problem: modem is hanging up without apparent reason. Connection may be sustained for up to three hours, but sometimes for only a few minutes. Same problem does not occur if the hardware configuration is used with another operating system.
Diagnosis: most likely cause is settings different in the modem. Your current chatscript file clears the settings back to saved defaults. The other operating system may be providing different settings to the modem.
Proposed Solution #1: find the modem initialisation string being used by your other operating system and use it in Linux, typically this would go into /etc/chatscripts/isp.com file for the connection.
Proposed Solution #2: there is a rejection of CCP by the peer, which can indicate suboptimal PPP implementation by the equipment manufacturer used by the ISP, so add noccp to pppd options in /etc/ppp/peers. This should prevent CCP from being negotiated. There is a 2% chance in my estimation that this will fix the problem.
Proposed Solution #3: remove the Z from the ATZ in /etc/chatscripts/isp.com so that the settings in the modem set by the other operating system will not be cleared, then connect using the other operating system, then connect using Linux, see if the problem goes away. This conflicts with #1 above but tries to do something similar.