Give these options a spin (take out the comments):
Option 128 = Mitel Phone System IP
Option 129 = Mitel Phone System tftp ip (or other tftp server in use)
Option 130 = Obvious, but not sure why it is needed.
Option 132 = vlan id (numeric) i = for integer notation
Option 133 = Priority for Mitel IP Phone - not sure what the heck that is for either.
dhcp-option=128,10.1.100.10 dhcp-option=129,10.1.100.10 dhcp-option=130,"MITEL IP PHONE" dhcp-option=132,4i dhcp-option=133,6i
Some folks have reported better luck with leases obtained without the vlan bit (opt 132)....
Example config:
dhcp-range=192.168.100.50,192.168.100.250,12h dhcp-option=125,"id:ipphone.mitel.com;sw_tftp=192.168.100.9;call_srv=192.168.100.9;vlan=100;l2p=6;dscp=46" dhcp-option=128,192.168.100.33 dhcp-option=129,192.168.100.33 dhcp-option=130,"MITEL IP PHONE" dhcp-option=131,192.168.100.33 dhcp-option=132,100i dhcp-option=133,6i