asterisk users - Jul 2010 - Pereserving the callerid value when presentation set to witheld over sip

We are a telco so when we receive calls via ISDN and the number is 
witheld we see the correct presentation value but also still see the 
actual callers number in the callerid(num) variable.

I am trying to forward some of these calls over to one of our other 
boxes via SIP but I have found that if the number is withend then the 
sip packet contains :-
From: "Anonymous" <sip:Anonymous at anonymous.invalid>

I am running Asterisk 1.4.30

Is there a way to work around this?
I would prefer not to have to prefix the number with '141' for example.

Thanks
Gareth