Hi Andy,
I'm afraid that CUCM does not have a field on its DB that will indicate if a phone is on a current call or not, if you are looking for an specific phone, you may open the web page for that phone and check the 'stream 1' page if that is active then the phone is currently on an active phone call.
Perhaps there is a 3rd party app that can give you this information.
HTH, regards,
Christian.