Insufficient resources error when try to create second channel-group on t1 controller 2921 router

[Alex Krivobok]

I have a 2921 router with a 4-port t1 controller. I need to add 1 more serial interface on a t1 port that is already in use but has unused channels.

However, when I try to add a new channel-group under controller t1 0/0/1, I get an error "Insufficient resources to create channel group"