You need to perform a few extra steps:
On the router,
voice service voip
h323
call preserve
then on the call manager (in my case 4.1)
Service -> service parameters -> advanced
Then under H323 set "Allow peer to preserve H323 calls"
After doing all of that try a test call and block access to the CCM via an ACL - phone should display "CM Down, features disabled" but the RTP stream should continue between the phone and router.