Dear Charles Park Seward,
That's an interesting test, and does seem to corroborate that the muting/drop-out problem is present on your phone, even though you reported being perfectly happy with it. However there's another test you can run, I'll repeat here in case it's difficult to find in this long thread. In that test, you'll hear it fail profoundly:
(1) Perform the test in a quiet room, so you can comfortably hear a phone call at the lowest volume setting.
(2) Insert the Apple provided headset.
(3) Call Hertz at +1 877 654 4400.
(4) After you initiate the call, lower the volume to the lowest setting (one square). (This must be done during a call because your call volume setting may be different than the volume setting that applies outside of a call.)
(5) Note the profound long silence after the call rings and answers - eventually you'll hear some broken speech.
(6) Call the same number from your good landline or mobile phone other than iPhone 5, and note that the long silence was actually a recorded message that your iPhone 5 discarded or muted so you could not hear it, and that the rest of the recording is not broken up when heard from another phone.
This test has reproduced the drop-out problem on every iPhone 5 I've tested, on every carrier, on phones whose owners claim work fine. Someone should pass this along to someone who can help us.