Try giving it blank username and password, i.e.
Username = " "
Password = " "
in wvdial configuration.
If that doesn't help, post your current wvdial configuration (if there are some things you shouldn't show publicly, replace them with something like *hidden* to show you've set them but don't want to reveal them -- obviously username and password are such things, but not in this case if you don't need them). I don't use wvdial with a gprs/3g modem currently, so my help is a little limited, but posting the configuration may help others to help you.