Do you remember the package you installed? That might have something
to do with it. My thinking here is that it might be causing you to
dial on bootup, and in that case I would recommend uninstalling it.

You should be able to find the /etc/init.d related in one of the
/etc/rc* related directories that is causing dialup to kick in during
bootup. My thinking is that uninstalling the package would eliminate
that, saving you from making any required low level edits (where one
always has to be careful if that approach is taken and you know what
you are doing).

I run with a USR 5686E external modem, and a 56k dialup account.

Although I run exclusively in a RAM-based (1GB) Live CD Hardy
environment (not from my install RH3 Fedora Linux - I do have it set
up the same), my setup is entirely using /etc/wvdial.conf, so given
some 8 or so phone numbers from my ISP for local access, I simply
issue the command:
$ wvdial phone1
where phone1 up to phone2 are configured for specific local access
phone numbers to my ISP.

Linux has traditionally required an external modem, and it sounds like
you have one that is external.

I would try uninstalling the modem related package, and setup the
/etc/wvdial.conf file for different local phone access numbers to your
ISP.