If you have access to the option 125 config tool it would be helpful. What DCHP server are you using? The process the phone goes through on boot is to use lldp, if unavailable use dchp to learned vlan, priority, phone system address and dscp values.
Generally the first dhcp request is on the untagged default vlan, the dhcp sever would have the necessary info and after the phone learns where it go it releases that lease. Then sends out a new vlan tagged one.
The tool creates both an ascii and hex representation of option 125.
The phone will also respond to option 43 as well as the old Mitel 128, 129, 130, 132, 133, 134 values (I may have some wrong numbers) but this is a quick overview from memory.
The 125 hex string starts with the mitel vendor code followed by a lenght of string value the the hex representation of ipphone.mitel.com;tftp_srv;call_srv,l2p and other values. These all get done by the tool. You may be able to look in you system to see the true format.
If using 43 you don't need the vendor id or string length values.