I've been tinkering around with a Cisco 7970 IP phone. After spending 3-4 days of trying to get SIP working on this device, I gave up hope and started working with SCCP. The good news is that it worked straight away!
While the concepts of sccp are a lot different than using separate SIP accounts, sccp is much more like standard phone system operating on lines instead of accounts. If you are struggling getting one of these devices working on Asterisk via SIP - save yourself the hassle and get it running using SCCP!