In all honesty socrates0, I didn't think you'd have continuing luck with your solution.
It seems to me that each time I try a different workaround to try solve this problem - whether that's a ringtone app or putting contacts in groups or whatever - it seems to work well for a short while, THEN, just when you think you've cracked it, it all goes wrong again!
I have tried countless suggestions now, but still the problem remains.
Why can't this be fixed???!!!