Hello,
You need to have the "IP Plus" feature set to run ITS in 12.2(11)YT, i.e, the image name must be "c2600-is" instead of "c2600-i".
I think the IP Plus image in 12.2(11)YT is available only for the 2611XM platforms with fast processor and more memories. Please see the following URL for more info:
http://www.cisco.com/univercd/cc/td/doc/product/voice/c_ipphon/keyswtch/ft_its21.htm#xtocid20
Do you have a 2611XM or 2611?
Thanks,
-ben