please check Redfone's foneBridge. i come across an article about this before.

On Tue, Sep 29, 2009 at 5:26 AM, SP <sprice@gmail.com> wrote:
Yes, with OpenSER/Kamailio/OpenSIPs/SER (you pick a name).

On Mon, Sep 28, 2009 at 15:12, Mike van Lammeren <mike@van.lammeren.net> wrote:
> Hello!
>
> I followed a tutorial that describes load-balancing Asterisk with
> Ultramonkey, but cannot get it to work with FreeSWITCH:
> http://www.danielaliaman.com/blog/files/ultramonkeyasterisk.pdf
> My X-Lite client fails to register with the server. I have looked at the
> packets with wireshark, and found that when X-Lite sends a SIP Register
> packet, it gets an ICMP response: Destination Unreachable (Port
> Unreachable).
> Has anyone got a load-balanced FreeSWITCH setup working?
> Mike van Lammeren
>
--
Shannon