power or the cable to an external modem is removed during the connection process
if the user cancels the dial/connection attempt prematurely
if the number dialed, including special codes such as to disable call waiting, are incorrect - check the number to dial and dialing properties (see Connectoids)
if you have call waiting and an incoming call comes during the connection attempt (disable call-waiting as part of the phone number to dial; check with your telco for the code to disable call waiting)
