WORKS! Make sure you are using a SIP Security Profile with only UDP for Outgoing Transport Type.
That was killing me till I found that on some other sites.
Here's the Trixbox side...
On the CUCM side, it's easy. Just set up a SIP Security Profile with UDP for Outgoing Transport Type and configure a SIP trunk with it. Make sure you have a Media Resource Group List with an MTP in it...at least that's what I read. A discussion of MTPs is too much for here. For my little set up with a couple of phones, the software MTP in CUCM will be fine.
I picked the defaults for nearly everything else on the trunk configuration.