Mobile phone microphones are unidirectional, meaning they will only record sound from one direction and a very close distance (around a few metres away). You could still get a good-quality recording by placing the phone on the lecturer's desk or table, but you will not be able to see the live captions.
The big disadvantage of using a mobile phone to record lectures/meetings is that it drains the battery quickly. If you’re recording a 3-hour lecture and you forget to charge the phone, this could be incredibly annoying (not to mention the battery could run out before the lecture finishes).
We do not recommend using a built-in microphone for lectures, as this will not generate the best captions possible for you. If you are speaking directly into the microphone from a short distance, the built-in microphone is fine.
Tip
If your only option is to use your built-in microphone, try sitting as close as you can to the lecturer/source of the recording.