Q: My son's phone calls appear on my phone as missed calls and visa versa how do we fix this -I have unchecked his number and email already but still this situation keeps occurring

When you say you share an apple account—I think that may be it.  If you have two Apple IDs in a Family Share group, that's fine.  But if both phones have a single Apple ID, then some things are going to be a bit strange.  Text messages will likely be duplicated between both phones and any associated computers.  Call history (which is partly shared with FaceTime history) will be unpredictable. If the 6S is set up for WiFi calling or Calling From Other Devices (an option that is offered if you turn on WiFi calling), which and where phone calls get directed may be unpredictable.  And from there on out it just gets stranger and stranger.

So the first thing to confirm is that you're using separate Apple IDs. Depending on how things were set up in the past, this may seem a strange distinction from email accounts, particularly since there's a strong chance they're identical.

It's absolutely fine to have multiple devices with the same Apple ID. But only do that when you want most everything—call histories, text messages, notes, reminders, iCloud drive, photo libraries—to be duplicated across all those devices. But when those devices need to have distinct lives—that's a big, baffling, knot to unravel, and it's likely to change behavior as updates provide new features.

I hope this helps!

Users should not share Apple IDs.  The preferred method is to use Family sharing instead:

Family Sharing - Apple Support

See the section"Avoid sharing an Apple ID" in:

Use your own Apple ID for Family Sharing - Apple Support