While flying, how do I contact someone who is on the ground?
Once, in a pinch, I persuaded Phoenix Approach to call my mother-in-law and advise her to meet me at an alternate airport since I would have had to hold for two hours to get into PHX where we'd originally planned to meet. This is definitely not something that ATC does on a regular basis.

Other options include carrying an Iridium handheld satellite phone (not cheap, but certainly a possibility for light GA) or installing an amateur radio transceiver.