Thanks. OpenAI's docs are frustratingly vague about the whole thing. It seems (assuming the 15 minute hard limit holds true) that I must have been conversing with advanced mode for 15 minutes since Advanced is the default for Plus subscribers on the mobile app, and then it must have possibly handed it off to the standard voice mode after that.
[1] https://www.reddit.com/r/OpenAI/comments/1hdamrm/so_advanced...