Call register showing on multiple iPhones?!?!

Hi all, okay this is driving me batty.
I've posted about this a while back in the Apple Discussion forums (as have many others suffering with the same problem) but sadly these days those forums seem dominated by a lot of self-righteous & less than helpful self-declared 'power-users' so I thought I'd dive in here to see if anyone can assist.

In our home & family small business we run two iMacs, three iPhones and an iPad (apart from Apple TVs etc)
Everything is updated to the latest version of the OS & iOS.
We share one Apple ID across all devices as this is the only way you can share multiple calendars, contacts, music etc without a lot of hassle.
For years this was never a problem until maybe 6 or 8 months ago. Then, inexplicably, messages & the call register from one iPhone started appearing on another.
A lot of other people reported the same issues around the same time.

After very long discussions with Apple Support & lots of fiddling around the problem just vanished as fast as it had appeared.
Now the problem has come back again for no explicable reason.
Some people on the Apple forums tell you this is a 'feature' and we should learn to live with it as 'it's the future...' others say ...'it's your own fault for sharing an Apple ID'...
Helpful. Not...

Here are the infuriating things (apart from the arrogant twits in other forums...) if it was a 'feature' it would be happening across all devices right?
Wrong. It's only happening between my iPhone 6s and iPhone 5s.
Not the iPhone 6, not the iPad, not the Macs.
The problem now is not sharing messages but sharing the call register. My daughter does not need to know every business call I have made or missed & likewise me with her buddies.

When talking with Apple they told me clearly on a number of occasions;
a) - this is not expected or intended behaviour
b) - it should be something that can be turned off
c) - there is no problem with sharing an Apple ID (they even tell you that it is REQUIRED for Home Sharing to function https://support.apple.com/en-au/HT202190 )

So, here's the thing... I put all four iOS devices down side-by-side and went through every single part of their settings. They are virtually identical (apart from obvious things like e-mail accounts).
That being so why is this not happening across all four devices? Or across none?

Here's what is turned off;
- Handoff
- iCloud Drive
- Calls on other devices
- iMessage set to the actual phone number of each handset-only not Apple ID
- Facetime set to phone numbers only not Apple ID (on the iPhone 6s I have even turned it off completely)
And probably other things I've forgotten.

I'm just baffled.
I can only assume it is some weird bug that crops-up under certain circumstances.
I should add that it only performs this over-sharing when both the iPhones are on the same wi-fi network.

Anyone have any suggestions?
Or is it something we just have to live with until Apple acknowledges it & sorts it out?