Could be their carrier, could be their phone is off, could be they rejected sending you a confirmation... I know I decline confirming receipt whenever I am able. My feeling, "Get off my back. I'll talk to you on my timetable, not yours."
Believe it or don't; some people find delivery confirmations pushy, rude, invasive and none of your beeswax no matter that you are the sender.
Have you tried having a conversation with the individual?