Chat channels
Just type — in-character, heard by people NEAR you. This is not global chat; the far side of the map can't hear you. /me <action> — proximity emote, italicised /do <description> — describe scene/state in proximity /ooc <text> — out-of-character in proximity /ask <q> — to staff queue, server-wide, persists until answered
Voice
Voice is proximity-based (pma-voice) — you talk to whoever is standing near you. To reach someone further, you need a walkie-talkie.
Walkie-talkie
A walkie-talkie inventory item gives you channel comms. Set your channel with /tune <channel>; everyone on that channel hears you. Without the item you have no radio at all.
Careful: /radio is NOT the walkie — that's the car stereo, and it only works from the driver's seat.