If you think no one is hearing you, check your radio control bar.
There are five channels in the radio control bar. The first four are for text communications. You will get any text, from other players, based on the four channels you have selected.
When you broadcast, by default, the text only goes out on the first channel. So if you have it set to the mission channel, then only those in that mission will get the text. Try setting the first channel to "Country" and that will allow you to communicate with everyone in your country.