Actually, I was going to suggest something similar as sixwheeledbeast. Recaller does that although it does it in a silly way (phone number first, which means makes mess in sorting by name). I was going to suggest the format YYYYMMDD-HHMMSS-phoneno.aac (where the 'phoneno' part is skipped if phone number not available) or, even better, make it configurable via a format string.