Hey saniyah170,
It looks like you're coming from the article: Record the screen on your iPhone, iPad, or iPod touch - Apple Support
That would have the best information for the steps to take to record your screen.
If the screen recording is stopping and not saving the full amount, we'd suggest checking to ensure you have enough available storage on your device to save the screen recording, as low available storage could be causing the issue. You can find steps to check and manage your iPhone storage here: How to check the storage on your iPhone and iPad - Apple Support
If that doesn't resolve the issue, you may want to provide some additional details about the situation.
Does the issue only occur when screen recording FaceTime calls or do you see the same issue with other screen recordings? That would help determine if the problem is app specific.
Which version of iOS is running on your iPhone?
Thanks!