wvdial.conf(5) has:
       Login  You must set this to the username you use at your ISP.

but if I use this, I see:
--> Modem initialized.
--> Configuration does not specify a valid login name.

The sample section, the default configuration file, and the files generated by
Red Hat's tools use Username instead of Login.

Changing the word "Login" to "Username" in my configuration file makes the
connection work.

Please fix the man page.