There's a kb on mol about vlan settings and 53xx phones on MCD 5.
Try removing the vlan and priority settings from DHCP option 125 and give the phone a boot.
Failing that.. Reboot the phone, hold the down arrow wait for it to present you options. Move through the options till you get to static network settings. Assign the gateway, ICP, and TFTP settings.. you can leave the tftp port blank or set it to 69, I think it's 69 anyway.