First make sure the phone is powered off, this can be done by unplugging the patch cable from the highlighted port below. The following has been done on a Mitel 5340 but the principals are the same.
Then plug the patch cable back into the phone, then quickly turn the phone over and hold down the number 7 key (keep it held down) until "Config Teleworker is Highlighted" as below
Then press the * key for "Yes"
Press # for "New" settings
Press * for IPV4
Then type in the address of your phone system, you can get this from your system administrator, or the Fusion support team. In the case above our address is 192.168.1.2, so we enter 192 168 001 002 (each number should be in 3 digit format). Then press the down arrow to save your changes. If at any point you make a mistake press the * key to delete it.
Press the * key to save your changes. The phone will then want to reboot at which point it should pick up a DHCP address and then download the necessary bin files from the phone system. Eventually it should display the below, ready for the pin code to activate. Please note depending on the model of you phone it may also say "USE Superkey key to send Pin".