Asterisk Sip trunking routing problem

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This problem is driving me crazy.
I have two severs which are trunked by the sip
Asterisk box A is natted thus in my sip.conf file I use the following
externip=DDNS address
Asterisk box B is NOT natted and has a static IP.
Asterisk box A and B are both registered with each other.
B holds the address of the DDNS public IP of A
for the sip trunk.
Everything works fine.
The problem comes in when on BOX A
I make a seperate PPPOE connection to route
only VOIP packets to BOX B.
BOX B still gets the DDNS address of BOX A
and not the new PPPOE address
This causes all sip requests to be rejected on BOX B
due to authenication failer
I traced it down the problem to the externip command in sip.conf
When I turn off externip, BOX B gets the correct PPPOE IP of BOX A.
I dont think I can just disable externip due to natting
as other sip clients need to access BOX A outside of
its network.
Anyone have any ideas on this problem or how to solve it