All voice chat in DOI is proximity based, and there is no "team voice chat". So anything you say in your mic can be heard by anyone, both friendlies and enemies, assuming they are within hearing range from you - just like in real life. This is the normal voice chat that everyone can use - no need to equip a radio.
The radio just works sort of like a "voice chat beacon". So if the radioman uses the voice chat, or if a non-radioman is within distance of the radioman and use the voice chat, then everything they say will be transmitted to all the other radios on the same team, and the voices coming out of the receiving radios can be heard by anyone who is within hearing distance - again, by both friendlies and enemies.
It sounds a lot more complex than it is. It's really very simple, I'm probably just bad at explaining - and English also isn't my native language, so that doesn't help either, lol. Anyway, I hope this made things at least a little bit more clear.