I remember receiving various of those "Please call me. Thank you." SMS or being "flashed" the last time I was back home. It appears that there are even certain "Rules of Beeping".
But beeping is not only about money. Donner's 'Rules of Beeping' suggests a social protocol for the practice.
"The richer guy pays," he writes. It is acceptable to beep someone if you are short of cash and they are flush with credit. Never beep someone poorer than you.
Never beep someone you are tapping for a favor. You don't want to risk annoying the person you are trying to win over. Never flash your girlfriend, unless you want to look cheap.Here is a Reuters article on the topic
The "Rules of Beeping" (PDF) from Jonathan Donner
Here is the study "Africa Calling" (PDF)
A Spiegel Online article on the same topic (in German)