What a nightmare I’ve had getting these two phone registered to FREEPBX. The 7940 with help from Dave on these forums but the 7971 was sheer perseverance and some help from a post by David Chidel, even buying the license to Endpoint Manager was no help.
So in the spirit of helping others, I’ll post both of my files here for others to benefit, just copy and change the indicated fields to your setup and you should be either ok or close to it.
Feel free to add yours 
Well I will once i figure out how to post it without the xml tags being stripped?
Email them to me and I’ll put them in the Chan-Sccp-B Wiki FreePBX Page. It probably needs some updates - I haven’t looked at it too closely in about six months. PM if you can’t figure out my e-mail address (Google “cynjut” and it should be pretty easy to find).
You have mail 
OK, so this forum doesn’t allow xml to display so I’ve put them on to some webspace for you all.
Enjoy