asterisk users - Jan 2005 - Peculiar one way convesation fault with Asterisk.

Dear Fellow Ast-Masters

Has any one here experienced the following issue (or similar).

Setup Description:
  - Asterix Server  - 192.168.1.10
  - Analog Phone connected through FXS to Asterisk Box - (ext) 2000 
  - SoundPoint IP 300 - 192.168.1.2 - (ext) 4004

Problem: 
I have a very basic sip.conf, extension.conf and zapata.conf. 
Now when I try using 4004 my call doesn't go through. But if 
I dial 4004 from 2000, it works like a charm. And, soon after 
I dial 4004 from 2000, if I try using 4004, it works fine too. 

Now if I leave it idle for a while and then try using 4004 to 
dial any number it doesn't work. The LCD on my soundpoint IP 
shows status as 'connecting' and then after a while gives up.
Whats even more peculiar is, if I issue 'reload' command in 
my asterisk server console it starts working fine. 

Problem solving attempts:
To find out more I started sniffing packets in asterisk server.
In ethereal this is exactly what I see,

Source 		   Dest  		Info
192.168.1.2	 192.168.1.10 	  SIP Request (blah blah)
192.168.1.10    192.168.1.2 	  ICMP: Destination Unreachable, 
				               type-3, code-10 (host 
                                               administratively prohibited)
192.168.1.2	192.168.1.10 	  SIP Request (blah blah)
192.168.1.10	192.168.1.2 	  ICMP: Destination Unreachable, 
				               type-3, code-10 (host 
                                               administratively prohibited)

<repeat>

After some reading I found out that this type of ICMP messages 
are normally sent out by the router when a host is blocked. The
document I read also adds, its mostly used in US government 
implementations(?). I haven't seen this type of ICMP message 
previously and not completely sure if I am interpreting 
it correctly. Are these messages even getting up to Asterisk 
server? If not, why not? if yes, then why is asterisk not 
accepting these messages?

Any leads or help greatly appreciated.

-r
PS: I have googled around, but to no avail.