This may be no more than an issue on the cell phone provider's (Verizon Wireless) part, though I'm told by them that it's my iPhone's configuration causing all of this...
But, anyhow, my iPhone 6s is taking three days to inform me of any phone messages left for me, whensoever. This has been a constant-occurrence for me ever since I updated my iPhone 6s to version 11.4.1(15G77).