Hi Ryan,
Sadly, this Intercom Button is not available on 7940's or 7960's. Here are the supported models;
Intercom Support for Cisco Unified IP Phones
Cisco Unified IP Phone Model
SCCP
7931
7941
7941G-GE
7942
7945
7961
7961G-GE
7962
7965
7970
7971
7975
Check Page 367 of this rather LARGE pdf;
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/6_1_1/ccmfeat/fsgd.pdf
Hope this helps!
Rob