Hi,
Assuming that you are using CUCM version 10.x or 11.x (as these versions require Cisco PLM and you can still purchase new Smartnet Contract). With PLM, the license files were never fulfilled using CUCM Serial Number or MAC Address. They are fulfilled using License Fulfillment request from PLM. So, I don’t think finding CUCM Serial Number is useful.
Before CUCM 9.x, the licenses were associated with MAC Address. If you can find the old MAC Address, you can reach out to Cisco licensing team for further help.
In either case, reach out to Cisco Licensing Team to find out the way to trace these licenses. Simultaneously, reach out to your Cisco Account Manager or SE to get the help.
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Thanks, Vaijanath S.