It appears this is not a new problem. It does however appear that it has never been fixed.
I have had this issue for about a year. I presumed it was my phone but have just had a new one and found out I missed a call on Friday.
NOTE: The really weird thing is that if the person calls a second time in quick succession it invariably comes straight through as a call. There doesn't seem to be any logic to when, where, or who. The only useful thing being that I can inform friends and relatives that it will happen.
I'm about to try the # code and sim removal fix and will keep you informed. But come on PLUSNET 3 years is a long time for this to be happening. I may ask for a new sim....has it helped?