Hi Todd,

Here are the notes for SIP SRST 4.1 and 4.0;

Prerequisites for Cisco Unified SIP SRST 4.1

Cisco IOS Release 12.4(15)T or a later release.

Cisco Unified IP Phones 7911G, 7941G, 7941GE, 7961G, 7961GE, 7970G, and 7971GE require firmware load 8.2(1) or a later version.

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_guide_chapter09186a0080874e61.html#wp1020952

Cisco Unified SIP SRST V4.0, Cisco IOS Release 12.4(4)XC

With Cisco Unified SIP SRST 4.0, a SIP redirect server is not necessary. Instead, a back-to-back user agent (B2BUA) server routes the call as desired. A B2BUA is a separate call agent that has more features than a redirect server, which can accept and forward calls only. With a B2BUA you can also configure call blocking and call forwarding. In call forwarding, the B2BUA forwards calls on behalf of the phone, while maintaining a presence as call middleman in the call path.

http://www.cisco.com/en/US/products/sw/voicesw/ps2169/products_configuration_guide_chapter09186a0080874e6d.html

Hope this helps!

Rob