I don’t know because when plugging 2 CUCM’s together you get a choice of building “regular” SIP trunks or “enhanced” ones that (presumably) are Cisco-proprietary and give you extra things. I’d assume the enhanced trunks would work and the regulars would not.
The issue I think is whether or not the call is entirely processed through the PBX. When you have a case with an endpoint registered into Asterisk and an endpoint registered into the CUCM and PBX-to-PBX trunks tying them together, my observation is when the call is connected it stays routed through the PBXes and PBX-to-PBX trunk. When it’s 2 endpoints registered into Asterisk and a call is connected, the call goes direct from phone to phone, and the CUCM works the same way. But when it’s a call into the IVR then transferred to an extension - who knows what happens. Your going to have to do some testing. If the IVR resides on Asterisk and it’s 2 extensions registered into Asterisk and they call each other via the IVR - then does the call end up phone-to-phone or phone-to-Asterisk-to-phone? It seems you have many test cases here you will need to chase down and see what happens.