Cisco SPA phones annoyingly get "reprovisioned" and reboot daily, by default every 24 hours from the time of the last reboot.
This is right royal pain in the butt and you have to edit the XML provisioning template to stop it.
Why not support the newer features of the later firmware revisions and allow via the 3CX console the setting of Cisco SPA 5xx phones to easily set a time of the day we'd like the phones to auto reprovision and reboot.
This feature first came out in 7.4.7 (released DEC 2010) and 7.4.9 is the latest SPA firmware as at Sep 2011.
See Cisco Documentation: