The first time you tap the record button, your phone asks whether Junior may use the microphone. Allow it and recording begins straight away. There is nothing to set up in advance, and Junior only asks once.

If you said no

Junior shows a screen headed "Junior requires access to your device's microphone", with a Device Settings button that takes you to the right place in your phone's settings. Turn the microphone on for Junior, come back to the app, and it picks up where it left off. You do not have to restart anything.

The Junior screen explaining it needs microphone access, with a Device Settings button
What you see if microphone access is turned off.

Questions

Why does Junior only ask when I start recording?

Because that is the first moment it needs the microphone. Asking at sign-in would mean asking for something you might never use that session, and permission prompts that arrive without an obvious reason are the ones people decline.

Junior says the microphone is unavailable.

The message is "The microphone is currently in use by another app or call, so we can't record right now." Something else has the microphone, usually a phone call, a voice note, or another recording app. Only one app can hold it at a time. End the other thing and try again.

Does Junior listen when I am not recording?

No. The microphone is only opened when a recording is running, and it is released when you end or discard one. There is no background listening, no wake word, and nothing is captured between recordings.

The microphone toggle for Junior is greyed out.

Your firm manages your phone and has set that centrally, which you cannot override from the device. Your IT team has to allow it. This is different from your firm turning off recording in Junior itself. If that has happened you will see recording is turned off for my organisation instead.