The server type should not be an issue as DHCP is a defined protocol, not a private one.
As such try the following string:
....Tid:ipphone.mitel.com;sw_tftp=(CALL SERVER);call_srv=(CALL SERVER);vlan=900;l2p=6v6s6( could just try it as 6); dscp=46v46s26 (or try v46s46).
Also convert it to hex, making sure the .... is converted as NULL hex instead of 2E 2E. NULL should be 00 00 04 03. If that doesn't work i will run it thru the Option 125 converter and see if that makes a difference.
Also you only need to define Opt 43 and Opt 125 for the phones, it's the paging adapter that really needs all the fluff (stupid paging adapter).
Another thought is to flip DHCP to the 3300 and then you can definitely point the figure at the DHCP server being mis-configured.
-Ian