No microphone permission: phone Settings > Apps > Protect by STRONG > Permissions — enable Microphone. Without it, you cannot speak through the camera.
Speaker disabled: camera Settings (⚙) > Sound Settings — confirm Speaker is enabled. When the speaker is off, intercom, siren, and audio warnings are all unavailable.
Microphone muted on camera: check that Sound Settings > Microphone is enabled.
Remember: the intercom is half duplex — hold the button to speak, release to hear. You cannot speak and listen simultaneously.