From my call-center days:
LMOM = left message on machine
RNA = ring, no answer (does that ever happen anymore?)
TBD = to be determined
Honestly, though, between the custom macros of MessagEase (my newfound graffiti/keyboard replacement) and the word-completion of TextPlus (a T9-type app with a customizable dictionary), I rarely find myself using abbreviations anymore; it's just as easy to get the full word in there, and that's easier and quicker to read.