I think will be more realistic if players will speak with nearest player using character voice. So if a player is near to you he can hear what you say but if is on another side of the map he can not hear what you say just only if is in your visual range (50m). This is just an ideea.
There must be a keybind (R) for push to talk, your words will be recorded on server, and only players in range of 50m will receive (hear) what you say.
Or there must be a predefinited phrases what you can chose for your caracter to say, (like in your caracter setup is a voice what say "I surrender"). Eg: hello, wellcome, need help, need food, I will kill you, comme with me...
By the way! In game text chat must be another colour not white! To differe from system text so you can see when someone say something.