Normal users cant use wvdial to dial a ppp conection, at least not by default.
Here are the perms you should set:
chmod +s /usr/bin/wvdial chmod +x /usr/sbin/pppd chmod +s /usr/sbin/pppd
Also make sure that
/etc/resolv.conf /etc/ppp/options /etc/ppp/resolv.conf /etc/ppp/peers/wvdial
are readable by users.
Other way to do this, in a more controlled enviroment is to set up a group called "dial" or "dialout" and give this group the above permissions