I suspect that you simply have a bad line. I used to work for IBM I have not heard of anything you can do to a ThinkPad modem to make a bad line better. The modem in a 600E is capable of 56kbps but I have NEVER seen one achieve that over the public switched telephone network. The modem will always (in conjunction with the modenm at the host) try its best but, in your case, its best seems to be 26.4kbps. That is not unusual - I have seen lots better and some much worse.
Terry, April 2006