Hi joeltish,
Thanks for reaching out to Apple Support Communities.
It sounds like your daughter is having an issue with the microphone on her iPhone.
We're happy to assist.
We recommend checking out the steps in the sections titled "Clear the microphones" and "If people can't hear you on phone or FaceTime calls" from the article here: If the microphones on your iPhone aren't working - Apple Support
Clear the microphones
1) Remove any screen protectors, films, or cases from your iPhone.
2) Clear any debris from the openings of your iPhone.
If people can't hear you on phone or FaceTime calls
1) Open the Voice Memos app.
2) Tap the Record button
, speak at the bottom of your phone, then stop the recording.
3) When you play back the memo, you should hear your voice clearly.
We’re recommending the steps above because they directly address the issue you initially posted about: people can't hear your daughter when she makes phone calls on her iPhone 8.
Should the issue continue, follow up with the steps in this article: If you hear no sound or distorted sound from your iPhone, iPad, or iPod touch speaker - Apple Support
Please let us know the outcome after giving the above steps a try.
Cheers!