Hi DRB1200,
Thanks for using Apple Support Communities. It sounds like you’re having trouble with your phone becoming unresponsive after locking it while on a FaceTime call. I’m happy to provide some steps that may help. If you haven’t already, I suggest restarting your iPhone and making sure it’s iOS version is up-to-date.
Restart your iPhone, iPad, or iPod touch - Apple Support
Update your iPhone, iPad, or iPod touch - Apple Support
If you’ve already done the above, the next thing I’d recommend is backing up your iPhone and restoring it to it’s factory settings to see if this issue persists. I suggest this step because you say that the same issue happened on your old iPhone, which means that there may be something within your backup causing this issue.
How to back up your iPhone, iPad, and iPod touch - Apple Support
Once you’ve created a backup, you can find steps for restoring here:
Restore your iPhone, iPad, or iPod to factory settings - Apple Support
Once you've restored your iPhone, you’ll need to set it up as new (rather than restoring from a backup) to test the issue. If the issue is not present, you can restore from your backup and test again. If once you restore, the issue is again present, we'll know it lies within your backup.
Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
Take care.
