limiting the port range Messenger can use for RTP?

I've seen a few posts in the Google archives about the port ranges MS
Messenger uses, but is there a way to limit then, so I need only open a few
on the firewall? I'm guessing not, and if I'm right, does anyone know if
this feature is roadmapped or not?

Also, when I register to an external SIP proxy,I see my client sets the
following in the SIP header:

Session Initiation Protocol
Request line: REGISTER sip:fwd.pulver.com SIP/2.0
Method: REGISTER
Message Header
Via: SIP/2.0/UDP xx.xx.xx.xx:8376

Basically, it seems to be telling the remote Proxy to connect back to my
machine on port 8376, which it does once I opened that on the firewall - but
why? I was expecting the standard port 5060!

Thanks in advance,

Jeff K