DCOM communication work across firewall with 'callback' support

Hi,

I want to make DCOM communication work across firewall with 'callback'
support.
Take a example,
I have a COM service A on one side of firewall.
I have a console application & a COM exe B on other side of firewall.
Using dcomcnfg, I have selected static port 5100 for COM service A.
Also for COM exe B, I have selected static port 5101.
And on the firewall, I have opened port 5100 & 5101 for both direction
traffic.

Now console application creates COM exe B object on the same machine & also
it creates COM service A object on machine across firewall. Console
application passes COM exe B object to interface method of COM service A.
This interface method call succeeds. But now COM service A tries to callback
on the COM exe object passed to it. But this callback fails with error "The
RPC server is unavailable".

Can someone please tell me what I need to make the callback work?

Thanks,
Sandeep