Yeah, I was looking at that and had the same idea. But maybe it would be better to put a trigger on UPDATE of that call duration column. As far as I can see, sqlite3 supports triggers, although I don't know if they work on the N900.
http://www.sqlite.org/lang_createtrigger.html

The trigger would be executed each time the total call time column is updated, calculate how much was added, and update the last call event with that duration.

Actually, I'm going to try that right now.