Why do keyboard sounds stop me choosing options on a call?

I have an android phone, and when calling through to any kind of customer service, it wouldn't let me select an option (the ones where it says "press 1 for X, press 2 for Y ..."etc.

I tried pressing the number, and i heard the keyboard sound, and saw the number on screen but it didn't register with the phonecall, and just had the automated voice repeating the dial message.

I tried everything, and saw a tip that said if I turn off the keyboard sound in the setting it fixes it - and tried it and it works, but everywhere I try to find *why* it works, all I can see is "it just magically does".

I'm not having a problem, becuase I can know get through any system with the press-a-number forwarding, but am really curious about why this works and what's happening to cause this!?