I don't know if it is me or note but I don't seem to be able to get Key Attribute Working on Outbound Calls. (v2018+). It's been awhile since I've configured it so I don't know if I have missed something or not.
I have created my set of fields (Demo Set 1 and 2 - did them under different Categories just in case)
But when I create an Outbound Call List Marketing Activity the option to pick which field set never appears