You should not have to put anything in the setup of the end-point, if it is defaulted, it will look for the options needed to boot from your tftp server.
If you have the DHCP Scope setup to support the IP segment, i.e. in the pool for the end-units.
Setup a scope option for 125 Mitel 125
Setup a Scope option for 128 Mitel TFTP, this points to the tftp server
Setup a scope option for 129 Mitel RTC
Setup a scope option for 130 Mitel IP Phone identifier
Setup a scope option for 132 VLAN for Mitel IP Phones
Setup a scope option for 133 Priority for Mitel IP Phone
If your not able to find how to do this, I will look up a document and post here. I am sure it is in the FAQ for Mitel website.
This works for ALL the end-points on the network, no need to configure each phone, unless they are off property and coming in to the network from a public IP, behind a public firewall. Then I had to setup the IPC IP