Public chat is opt-in only. Bungie emphasized the importance of consent in its philosophy on the feature. “We believe that requiring players to opt-in to chat leads to more friendly environments,” explained design lead M.E. Chung. The feature goes live as part of update 1.0.3 on November 18.
Whenever you join an activity in which voice chat is enabled, a message will pop up in the lower left corner of the screen prompting you to opt in to the channel if you want. By default you must manually opt-in to the Team Chat, but in the Settings menu you have the option to set it up such that you automatically join when playing solo, without a Fireteam.
If you decide that public chat is not for you, then it is always possible to revert back to Fireteam Chat through the same Navigation Mode menu that brought you in. The current channel is displayed in the lower-right corner and you can toggle between them with the directional buttons.
