1.For old dialup networking code. If you are running win 95 (win 98 has the latest version) please update to Dun 1.3 go to ftp.recorder.ca Then to Pub, Then to Win9x, Then to Dialup Networking and download MSDUNN13.exe
2.Also try adding under the Modems in the Control panel under properties/connection/advanced/extra settings add the init string S15=128S10=40 this will sometimes help out as well
3.Sometimes feedback from other phones on the system will also cause problems. Try disconnecting all the other phones on the same phone line and see if you get the same problems.