Ok, i do the following and all now seems ok (i'm on debian):
1) uninstall snomone (dpkg --purge snomone)
2) remove snomone directory (rm -fr /usr/local/snomONE)
3) reinstall last version with dpkg
4) reset of snom300 and shutdown (*,vol up,*, vol down, #)
5) modified the default group 72 to use "external" ringer
6) modified with the webpage took the pnp -> ringtones.xml to use a custom ringer for external instead of the predefinited one
7) power up the snom300 and now everything works fine