Sangoma had a very nice article - not written by them but written by a community member - that detailed how to add a Cisco 3PCC phone into FreePBX on the Sangoma Help (digium.com) website. But that article is now missing - even though many other articles are still present. In fact, searching that site for the keyword “3PCC” brings up many Cisco phone related articles, but none involved with 3PCC.
I believe they have their website marked to not permit this. A number of sites do this they put headers in their sites that the Wayback machine respects and will not then archive the site. Other companies routinely issue takedown requests for old entries of their sites in the Wayback machine. It is part of campaigns to remove servicing documentation and software for older products in an effort to force obsolescence and encourage the customer to upgrade. After all “Why be you when you can be new?” lol