You would need to look in first place for what common protocol can be used by both systems, either SIP, H323, QSIG, etc.
If they have a common protocol, then you would need to configure the necessary trunk between them, to enable calls between both systems.
Yes, you might be able to place calls from your other PB thru Cisco, assuming you can trunk both systems with a common protocol.
You can search in the Cisco interoperability portal for references if Cisco has tried this, or reach your SE.
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