For those who were searching the solution

All directories into /sms/ are created with 'sticky' mode, so, if, like me, you try to manage incoming files with a web server, even if you put 'www-data' as group, nothing happens when you want to delete incoming files and not treating them again and again. Only the owner, smsd, has the right to delete. So, you have to suppress the sitcky mode on every directory you want to delete files.

chmod -t /var/spool/sms/incoming (for exemple)

And it works fine