Hey there Galliganhunter,
Thanks for reaching out to Apple Support Communities. We understand you’d like to know if LED flash alerts will work with FaceTime. We’ll be happy to help.
The article for LED flash alerts mentions this is for when you receive a notification. To verify, has this ever worked for FaceTime? If so, and it recently stopped, try turning the feature off, restarting, and then enabling it. These links can help:
Get LED flash alerts on your iPhone or iPad Pro
Restart your iPhone
You may want to make sure your device is up to date as well:
Update your iPhone, iPad, or iPod touch
Take care.