On Wed, 06 Jul 2016 12:18:00 -0400, J.B. Wood wrote:

Hello, all. In addition to cellular service I still maintain a
copper-back-to-central office phone line at my residence. I've had this
for about 40 years and never had any problems - until now. Here are the
symptoms (same behavior on any one of 3 extension phones):

1. Phones ring as expected in response to calling party 2. Called
party (me) can hear calling party 3. Calling party can't hear me 4.
DTMF (Touchtone) phone keypads produce audio tones but won't break dial
tone.

I've got a call into Verizon on the above but am scratching my head as
to what might be causing all of this. Your thoughts would be most
appreciated. Sincerely,

I experienced precisely the same thing due to corrosion on the
connections at entry causing it to behave as a rectifier.

Cleaned up the connection and all was well.
Took me a week to find it.
YMMV