Make sure that you have the latest software on the phone, or at least 1.13.
During the call you can press MENU and then select "ACTIVATE DTMF".
Now the Konftel will send the DTMF tones in the talk channel.
You can activate this feature permanently by going to MENU > SETTINGS > DECT > INBAND DTMF.
On Alcatel Enterprice you can use the *# command to enable DTMF.