If you've got multiple phones having this issue once their rebooted, then I have some test for you.
Start my statically forcing a phone to the voice vlan, by holding the 9 key on boot.
If that doesn't change anything, statically set IP Address, Netmask, Gateway, ICP, TFTP, VLan, and Priority on the phone.
If that does work and the phone downloads, then you've got a DHCP issue. My experience has been that the data DHCP source is somehow answering broadcasts on the voice vlan now as well. Usually means someone's moved the data DHCP source to a different switch port, that has both vlans on it, when it should be data vlan only.