Limited-access users can't send 1:1 voice messages unless they receive one first; they must receive a voice message from a specific contact to be able to send a voice message to that contact.

If a limited-access user tries to send a direct voice message unprompted, they will receive an error message on the PTT screen.

This setting is particularly useful for team and hidden channels, both of which automatically add direct contacts and enable 1:1 messages. Please note that not all channel types automatically enable 1:1 messages. Learn more about channel types. 

Limiting a user's access requires the admin take two steps: turn on the Enable limited access users function in the console, then toggle off an individual user's unrestricted access setting.