well, ive been looking around and it seems ubuntu people have been having problems with ppp altogether regardless of ISP, not exactly sure why other than maybe some of ubuntu's automated networking is interfering somehow
http://www.ubuntulinux.org/wiki/DialupModemHowto
check /etc/pppusers or wherever that file is and see if its got an entry for you in there, also check /etc/group and see if there's one for ppp
also follow the steps in that wiki I linked above and see what happens