asterisk users - Jun 2008 - Bridging an existing PBX in with Asterisk

Have any of you worked with a Fujitsu J5600 PBX before?  How about connecting it
to a Asterisk server?  Currently this customer has two pri's connected to
the Fujitsu pbx  box.  I'll be introducing Asterisk into their environment,
and will slowly migrate all users to Asterisk over a period of several months. 
What is the best approach to this?  Should the PRI's remain connected to the
Fujitsu box or move them to the Asterisk server?  Should the PRI's be split
between the two?  How about connecting up Asterisk to the old PBX?  Any help on
this is great appreciated!!

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