I would first look at DHCP to see if Option 004 (Time Server) or Option 042 (NTP/SNTP server) is set to tell the phone where to get it's time setting.
The Location settings under Regionalization also have a setting to tell entities whether to get their time from the IC server or another source, so check that setting in IA as well.
Also, on the phone itself, press the Menu button, Status, TCP/IP Parameters, and scroll down to the SNTP Address - that should tell you where the phone is getting its time from.