We are having some issues with UC520.  Various IP SPA508g phones will not allow outside calls on PRI.  The phones will run fine then all the sudden no go.  You press 9 dial the number and you get dead air.  If you reboot the phone that seems to fix the problem for a short while.  Then we got issues the VM pilot DN3099 same deal some phones go through to VM some do not.

I am getting tired of rebooting phones.