asterisk users - Sep 2008 - PRI incoming call forward / call redirect

Good morning,

I have a Bell Canada PRI here (switchtype=national) and I am trying to perform 
a call-forward-unconditional on one of the DIDs.

The idea is that when DID 5551234 receives a call, Asterisk redirects it back 
out the same PRI to some external number.

This is simple enough to do with something along these lines:

[PRI]
exten => 5551234,1,Set(CALLERID(RDNIS)=${EXTEN})
exten => 5551234,n,Dial(Zap/g1/5556789)

This is a brute-force approach but there are two problems:

1) it's not a true call forward
2) RDNIS does not appear to be getting set (i.e. the remote box with 5556789 
as a DID does not seem to see RDNIS

I'm not overly concerned about 2BCT capability at this time (it *is* talking
to a 5ESS although I'm not sure if Asterisk will attempt 2BCT with
national-2
switchtype) but it is important to be able to retrieve RDNIS, as the hope is 
to redirect a number of DIDs to one external number, and have the external 
number "see" which the original number was through RDNIS.

I had this working great the "other way" -- some external POTS number 
call-forwarded with *72 to a DID on this PRI, the DID saw RDNIS just fine, 
but now I'm trying to go the other way round.

Any ideas?  This is Asterisk 1.4.18.

Regards,
Andrew