Hey Tim800,
It appears that you're having a problem using the speakerphone while on a phone call. When you try to switch to the speaker, the screen goes blank a few times before you're able to tap the setting. We're glad to glad to provide assistance.
Restart your iPhone - Apple Support -- See if restarting your iPhone resolves the issue.
If it doesn't, find someone who's willing to test this with you. Remove any case and/or screen protector. Call the other party and use the receiver for a bit. When you pull your iPhone away from your ear, does the screen still flicker? If it does, Force restart iPhone - Apple Support. Once you're back to Home Screen, call the other party again and test.
Let us know how that goes.
All the best.