David,
Call Manager will route on the most specific match but also based on partition order in the CSS.
So if you have
CCM_phones_p
nortel_phones_p
externalcalls_p
outboundcalls_css - ccm_phones_p, nortel_phones_p, externalcalls_p,
extension 2000 in ccm_phones_p
extension 2001 on the nortel
route pattern 2XXX in nortel_phones_p to Nortel gateway
a user who calls 2001 from a cisco phone with the outboundcalls_css assigned will go to the nortel, but dialing 2000 would go to the CCM phone, firstly because it is the more exact match and second because of the route list order.
Hope this helps.
Paul