|A quick google search yielded not only your similar post on another forum but also the following:|
603 Caller's buffer is too small.
I suspect you need to increase the buffer size in the modem's init string.
But seriously, don't ask me how to do that. 10 or 15 years back I could have told you but I haven't touched a dialup modem since the inception of highspeed.
However, you can figure that out yourself by researching (using google) to get a manual for your modem which should detail the different settings for the init string.
Or, you could try the advice of the guy on that other forum and just reload the drivers for the modem.