Problem: A Cisco IP phone (in my case 7970G) does not sync with an NTP server. The phone sends an NTP request, receives an NTP response, but does not set its date and time.
Solution: It’s a known bug of Cisco’s IP phone software releases 8.3(3) or higher according to Cisco bug id: CSCso40588. Cisco says that these phones should sync their clock via a “Date:”-header in SIP packets, and NTP stays broken. Another info is, that 8.5(2)MN1.2 fixes this bug. We will try this.