Eugene,
Bell will configure PRI's with up to 5 groups (per PRI or across the
two PRI's on one PM3) and each group has a phone number assigned.
Set up a separate group for your ISDN callers but don't have it hunt
into your Centrex lines.
I suspect it's possible (though it might take some convincing of the
Bell programmer!) to configure two "groups", each using ALL the lines
in the PRI, one hunting and one not. Give one number to your ISDN
people and the other to your modem callers. I haven't tried this as
all our ISDNs are dedicated and therefore have specific numbers to
call.