I have been working with a similar setup myself, but for some reason I ended up ditching the
UltraMonkey setup because I just couldn't get it to work right.
It's been quite a while since my effort, so I don't remember what the exact issue was.
I got registrations to work, but had some other sip-dialog issues.
We have since then changed over to running OpenSIPs as a loadbalancer in front of
multiple FreeSWITCH instances. This setup is still in testing, but seemlingy works fine
(and if it doesn't, it's my own fault for writing a bad opensips config).
After we have done some more testing I can create a wiki-page with config details.
Best regards,
Even André