On your system, the programs tip(1) and cu(1) are probably executable only by uucp and group dialer. You can use the group dialer to control who has access to your modem or remote systems. Just add yourself to group dialer.
Alternatively, you can let everyone on your system run tip(1) and cu(1) by typing:
# chmod 4511 /usr/bin/cu # chmod 4511 /usr/bin/tip