Hello Bernt, thanksss, the solution is near....I´ve copied a script and type two lines at the end
sleep 15
sudo wvdial
Then I´ve named it modem.sh , chmod 755......., update-rc.d modem sh defaults and when I reboot the raspberry, it connect¡¡¡¡ It´s a great step.
For stop wvdial I´ve typed in command line sudo killall wvdial, the led of my modem turn off, then I´ve typed shutdown command for turn off the raspberry but during the process the led of my modem turn on and the raspberry turn off 4 minutes later, is normal??
Answer your question, I tried something similar but I´m sure that I didn´t remove /etc/network/interfaces part.
Thank you very much one more time