[whpalmer4]

Call Joe Smith about XYZ @Agenda:Joe Smith Start:today Due:today
found either by looking at actions starting today, or actions due today

or,

Call Joe Smith about XYZ @Calls
found by looking at your @Calls context which you somehow magically know to check, and then check the Agenda:Joe Smith context while you are on the phone to make sure you've covered all matters concerning Joe.

Many of us find this scheme works fine, so long as you don't throw up too many theoretical roadblocks to convince yourself that it cannot :-)