my iphone 6 does not ring when I receive a call...only vibrates Help

Check you didn’t turn on do not disturb and the mute button on the side of the phone wasn’t muted

Or you can Force-restart your phone

1. Press and hold both the Sleep/Wake button and the Home button.
2. When the Apple logo appears, release both buttons.

https://support.apple.com/en-ca/guide/iphone/force-restart-iphone-iph8903c3ee6/ios

Do you have the ringer switch turned on?

Perhaps this ASC post: https://discussions.apple.com/thread/8510454?language=en will answer your question.