Even though you will not actually hear the DTMF tones on your side when pressing the keypad buttons in Express Talk during a call, please be assured the DTMF signals are still being sent over on your VoIP line to the destination. It should act like any other telephone keypad in a call. If you are experiencing difficulties, it could be that for some reason your VoIP service is not receiving the dialed digits and relaying them on.
One other thing to point out is that on digital VoIP networks, there are 3 main ways to send the DTMF tone signals:
inband, sip info, and rfc2833. I believe (although could be very wrong) that NCH products only support inband DTMF signals, and hence the problem may be that your VoIP provider only accepts one of the others.
Also, you cannot get a refund for the product once you have activated/used it. NCH offers trials and/or additional free versions of every product for you to try out before you buy. NCH suggests that you don't purchase until you are sure all parts of the software are functioning correctly with your setup.