I'm having the same type of issue but since we use 2 vlans (one for voip and another for data) on a switch reboot, it seems that the phones pickup an IP from the default vlan of the data DHCP instead of the higher vlan id for the voice.
I suspect this is the same type of issue you are probably still dealing with, since a POE reboot (disable-enable) always brings the phones back to the correct voice vlan.
Did you resolve the issue with the Cisco SG models or find another POE switch that does a better job on reboot?
Wish I knew this issue before deployment!!!!