PPP can only be started from the "root" ID. ("PPP is not installed..."). Even using another ID with the "su" command won't start PPP (modem initializes and then PPP is aborted). I remember reading this in a FAQ once, but at the time I read it PPP wasn't installed. Now I can't find that information anywhere.
I'm using RedHat Linux Pro 4.1a. I also have Linux 4.1 (Vanderbilt) on-hand, but it isn't currently installed (if you have a solution that only works in Vanderbilt, I'd be willing to switch Linux versions).