I assume I have to add this ext. to a ringing group to answer incoming co calls without pressing a button. What manual do I need and is it doable on set programming or only through terminal programming? Thank you.

I suspect you will need database programming software one thing you can try is SPCL 324 and watch the display if it says accept HG calls your lucky and the phone should ring and you can answer by lifting the handset. If it says Divert HG calls you will need to do it again and call your Mitel dealer for support
Randy