T46G and Plantronics HW251/A - no outbound audio

[myoharry]

Hi all,
we want to buy new Yealink phones and tested our existing Plantronics HW251/A headsets connected by cable U10P-S with the T46G and T48G. You can hear the speakers on the other end of the line, but they can't hear you. It looks like the microphone is not working properly.
I saw another very old thread descibing the same problem with the HW251N, but there was no solution posted. Is this a known problem and is there any solution?

Thanks in advance for your help.