I am facing same issues with nothing phone2 and nothing 3a…
Answer from nothing support team:
Until then, you can try this quick workaround. During a video call:
• Swipe down from the top-right corner of the screen to open the Control Centre (you’ll see three green dots there).
• Look for the Microphone Mode option.
• Tap on it and switch from Voice Focus to either Normal or Environment mode.
• Then check if your child’s voice is now coming through clearly.