The lookup from the handhelds are coming from the exchange GAL if your BES is configured properly in mappings for organizer data synchronization.
Try to go to Blackberry Solution Topology.. Blackberry Domain.. Component View.. Synchronization.. then click the mappings for organizer data synchronization tab.. then click edit component (bottom of page).. and change which ever field has the PIN. I think its user defined string 1.
ps- i've not tried this so am curious to know if thats it.