"The port was disconnected."

This modem error can be a bit vague... since we don't know if your computer/modem initiated the disconnection, or the remote computer/nas/ras disconnected you. This error message could be caused by the remote computer/nas/device disconnection you, which is Error Code 629

I'd start by checking all of the settings in the connectoid for your ISP. Also check the modem settings, make sure its the right modem listed, and the right speed (ie not to low), and last but not least, check the modem init string, or lack there of). You could use a modem init string to limit your modem to a lower more stable speed possibly, just enough to get online, so you can download newer drivers for your modem from your modem manufacturer.

There could also be a problem with the line, look for damaged phone cords, use a different phone jack/phone cord, try a boring analog phone on the line and try to listen for static/noise/any sounds that shouldn't be there.