Hello everyone,
I've juste tried the solution of pbx support by changing the line :
<admin_mode perm="">off</admin_mode>
by :
<admin_mode perm="">on</admin_mode>
on the file snom_820_phone.xml.
After some searchs, this template is also used for snom821, snom760, snom720.
I rebooted my phone and the admin access is granted !
Thanks for the help. I learned something new today.