Under Debian GNU/Linux you need to add user to special group called dip so that dialup software (such as kppp and pppd) can be used by normal user.
i) Login as root user.

ii) Add user jimmy to dip group:
#usermod -G video,desktop,dip jimmy

iii) Add user jimmy to /etc/kppp.allow, so that user can dialout:
# vi /etc/kppp.allow

iv) Append/add user name jimmy and save file & exit to shell prompt.

v) Setup proper permission:
#chown root:root /etc/kppp.allow
#chmod 0744 /etc/kppp.allow

vi) Setup account, username, password, phone number etc by running kppp as root user.

vii) Login as jimmy and dial out.