Small update:
Decoding the voice-mail files (*.rec) to wav format was already found months ago. (yeah, sometimes I
do work on the things I plan to do)
One of the last 'problems' I had was linking those voice-mail files with the phone calls from the call list.
I think I've got that one figured out too now. I found a set of files that seem to contain all the information I need.
Dates, times, phone numbers and voice-mail filenames, although some of the data is not in a human readable format.
Time for some serious binary file dissecting
BTW this doesn't change anything to the time table (cause there isn't any

)