Phones are just instructed to listen to X multicast IP/port and really never know or care who is the one multicasting as long as it's in a codec they understand. Theoretically it should work assuming you're blocking the CUCM stream in your network and just allowing the one from your other device.
If you properly configure your MRGLs it should work for any call (again, just in theory)
HTH
java
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www.cisco.com/go/pdihelpdesk
HTH
java
if this helps, please rate