I'm setting up a intercom using the integrated intercom line in CallManager 6.1.3 for several phones-7961, 7962, 7965 and 7975. The intercom will come up with no problem on the 7961 & 7975, however I get a blank line on the 7962 & 7965 Cisco IP Phones. The 7962 & 7965 are running SCCP42.8-4-3S & SCCP45.8-4-3S, respectively. Do I need to upgrade the firmware? How can I get the intercom up and running?