Note, the actual hardware MAC address of a phone device (and some devices will have multiple MACs for wired and wireless interfaces) is not visible to CUCM, and so can't be retrieved in any way.
If you are referring to a phone's device name, which often includes the wired ethernet MAC in the device name - e.g. 'SEP123456789012' - then you can retrieve that info from AXL via the <listPhone> or <getPhone> requests. However, note that the device name for many device types (such as Jabber softphones) typically do not include the MAC as part of the name - e.g. 'CSFDSTAUDT'.
In order to get a list of phones based on part of the phone description, you can use <listPhone> as below, where the '%' character is a SQL wildcard indicating 'any number of characters':