T54W - Hands free mic not used if handset is hanged up

[Hexanet]

Hello,

A client encounter a specific trouble when he answer call with the free hands option.

The mic used is the one of the handset until the handset is repositionned on the phone.
After that the mic used is the speaker one.

Is there any option to activate these two mic ? or disable the handset one when the client push the free hands button ?

Thanks for your help.