There are lots of bits of information everywhere, but no single source, and this was one of the hardest to find, though the answer is simple ONCE YOU KNOW !
First, make sure your phones have the latest firmware and can boot from tftp cfg files - see my post here :
FreePBX / Asterisk should be set up for this out of the box. The magic comes either in the phone web panel, or in the cfg files
In the web panel go to User List Config and select the relevant configured user.
Look for : BLF User Group List
In the box you need to add the User numbers you want to have indicated, separated by commas.
Next go to Key Programming.
The phones see to be quite particular about how the keys are programmed. I got mine in a knot and then had to reinstall the firmware/config to clear up the mess !
Select a key. Set the Feature to Speed Dial+BLF. Set the context (Device number) and give it a label - careful on the letter limit - I am not sure how to separate the Label from the Description so to be sure I kept to 10 letters
Now add the number of the extension this button will call and set the Type to name/number.
Save and exit.
Now when the user of that extension picks up the phone their light should come on.
If you use config files you need to add the following :
For the 5215 you have 7 keys so with 2 line keys that leaves 5 extensions
With the 5220 the numbers are 25-38
25 is bottom right. 26 is bottom left. Columns work from the bottom up, odd on the right and even on the left. I chose to leave my 2 line keys where they were at 25 and 27.
To tell you what the Feature codes are you should refer to this document if you can find it : http://edocs.mitel.com/UG/EN/DK115611_16.xls
Alternatively try experimenting with different key codes and then try to the Config download to see what codes are created.
And presto. Happy users with flashing lights.