When trying to preview wallpapers or ring tones on a 7941 phone your receive "An unknown error occurred on your Cisco IP Phone". Authentication with a user / password appears to work fine at loggin because the available phone list is updated appropriately.
It appears in my case the CUCM Authentication URL was incorrect. Typically it's http://YourCUCMserver:8080/ccmcip/authenticate.jsp by default (7.1.X). Whoever was testing the IP paging solution here never set it back to default and all push technologies likely failed.
Check your Enterprise Setting URLs, specifically the Authentication URL. Remember to to reboot phones after modifying this entry to have it apply.