First of all check the dhcp configuration i.e network, Gw and tftp address.
Try with manual configuration of ip phone settings and static addresses.(if u have less phones)
Try restarting or resetting the phones, i faced a similar kind of issue with 7945 phones when reset them they started working fine. But first make sure you have the proper phone loads resetting some time gives issue with the phone loads and could be worse if phone unable to reach the tftp server.