asterisk users - May 2005 - Restrict RTP ports - inbound and outbound?

I have asterisk working great on my LAN but cannot open the necessary
RTP ports on my ISA firewall server to get to the outside world.  (I
must open each port manually due to an ISA limitation, and opening 10000
ports will takes weeks).  Ideally, I would like to reduce the number of
RTP ports in use.
 
I noticed in rtp.conf that I can limit the range of RTP ports.  So:
 
1.  How many ports will I need for a home with 3 phones (and two
external phone "lines").
2.  If I restrict the RTP port range to 10000-10005, does this mean that
my asterisk server will only use these RTP ports for outgoing, but an
external host may still try to contact my server on any port between
10000-20000 if the call is initiated externally?
 
I tried restricting my RTP port range to 10000-10005 but on startup
Asterisk is complaining about running out of RTP ports.
 
Thanks,
Mike
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