People can't hear me when I call them. Something to do with DTMF tones?

[nside]

I've noticed that when I call people occasionally they cannot hear me. I don't think it's a reception thing because if I call back the same thing happens. I've noticed that this happens mainly (if not always) when I call businesses. Businesses usually have call menus (press 1 to go here, press 2 to go here)... The tones (DTMF) work just fine but when I get to a live person they can't hear me at all but I can hear them just fine. Does anyone else have this problem?? How can I fix it? Thanks!