Are you saying that the same config sometimes results in 403 and sometimes 404? That would be very strange.
Possibly,
will help.
You probably need
Auth Username: +3908********@ims.vodafone.it
If you still can’t register, post the output from pjsip logger for an attempted registration. Mask your username, public IP and anything else you consider personal, but make it clear what each redacted value represents.