I tend to use the Android version more often and find that it has the same annoying habit. If I'm in a quiet room, the initial DTMF sounds, when dialling are quite noticeable to anyone nearby. I have not come across anything that disables that, and I agree, it is not necessary as a simple click sound would suffice. Perhaps someone from 3CX could comment, or, another Forum member has some suggestions.
You might consider editing your subject line, to solicit more of a response, as the term "dialtone" has a different meaning compared to "DTMF tones", for most users