SIP Trunk between CUCM 8.6 and Trixbox v2.8.0.4

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.