Yes turning off G722 in the handset did the trick.
To summaries: -
I installed the FreePBX-64bit-10.13.66.iso distribution.
We have Yealink T23G handsets. By default G722 is enabled.
Using the default sound pack (English) with G722 enabled works fine.
If you swap the sound pack on FreePBX to English United Kingdom, you get silence if the handset is using G722.
If you disable G722 on the handset English United Kingdom sound pack works.
I will also post my findings in http://community.freepbx.org/t/fpbx13-rc-g722-and-1-way-audio-even-to-voicemail/31042/36
A supplementary question about codecs.
Which codec is the best to use to avoid trans coding ?