asterisk users - Jan 2004 - Using the FLASH key on MGCP sends DTMF to the third party

This case happens on a Swissvoice IP10S phone. A key (F1) is defined as 
FLASH, which allows to transfer a call. This function works well, but 
the other phone will hear the DTMF keys, while the voice is muted.

Any idea how to mute DTMF as well ?