I don’t know what you mean by “secure”. Do you mean a voice chat that is moderated or a voice chat that is private, safe, and encrypted?
Under the assumption that you mean the former, I just thought of something that could work.
The Roblox system could record all voice chat activity on every game for moderation purposes. These logs would usually be ignored unless someone reported an account using this. If that happens, a Roblox mod would look through the audio recording of the voice chat and would decide the punishment for the user.
This system is the best thing I can think of, but this system is not perfect. There are some consequences with this system, such as privacy concerns and the possibility of poor moderation.