I got the idea of an app that could show you, after a telephone call, what percentage of the time you were speaking and what percentage of the time you're conversation partner was talking.
For this I had two conceptual approaches, and I was wondering which of the two was most suitable for this application.
You couldn't get access to the phone conversation data itself, but you can get the up/download statistics. There are a few apps available that track 3g data by other apps (most phonecalls now a day use 3g). You could start tracking this when a phone call comes in, filter the other app datatraffic out, and then end when the phone call ends. This way you can know the amount of traffic being send in & out. You would have to do some basic testing beforehand, as there will lot's of overhead data being transmitted besides the voices themself. You could do some testing to see if there is a correlation (and by what extend) between the amount of talking and data traffic (there should be).