wvdial as normal user

[zen_guerrilla]

Hello all,
I can't run wvdial as non-root user. So far I have done the following :

- 'chgrp dialout' & 'chmod g+wr' these files :
/etc/wvdial.conf
/etc/ppp/*secrets
/etc/ppp/peers/*

- chmod u+s /usr/sbin/pppd

but when I run wvdial it returns :
--> WvDial: Internet dialer version 1.53
--> Cannot open /dev/modem: Device or resource busy
--> Cannot open /dev/modem: Device or resource busy
--> Cannot open /dev/modem: Device or resource busy

/dev/modem is a symlink to /dev/usb/acm/0 since I a acm usb modem...

The same configuration on my redhat 7.3 worked fine. Any suggestions ?