Well on that advice had to try it
And you spotted that one, test subject binatone cordless, circuit built
used 2.2uF rather than 3.3uF , because was all had to hand.
Didn`t manage to try out chat cord software because needs XP , test PC
running ME, using
https://www.voiptalk.org/products/really_useful_signup.php,not an
affiliate link just a free UK geo number, running on Xlite, works very
well with a headest, works, kind of, with the adapted binatone.
As you anticipated , tests to mobile phone from binatone>adpapter>PC
resulted in a loud delayed echo being delivered back to the mobile,
call clarity on the binatone was fine.
Some twiddlling with mic gain did appear to make some differnce, but
also wondering on other factors, wonder if chat cord software has any
form of echo cancellation built in, main feature appears to be to
recognise DTMF tones from keypad.Will try adapter on XP installed
machine.
Is the substitution of capacitor value critical?