I've linked systems, but never a Cisco Call Manager.

The PRI was split into two groups: one configured as a regular trunk group with B channels, the other group configured as OPX stations.

For outbound calls, access the PRI trunk group and dial the Cisco extension number. Either put a trunk group key on the phones, or number the trunk group with a single digit (let's say 6). To dial Cisco extension 123, press the trunk group key then 123 or simply dial 6123.

For inbound calls, the Cisco users would do the same thing. Dial the trunk access for the PRI, which gives them dial tone on an OPX station. They simply dial the 5000 extension number from there.