You are correct - there is no API that would allow an app to intercept a phone call, either incoming or outgoing (you'd have to intercept, let the app do something, then pass it back - all in a matter of milliseconds, of course). Microsoft is the only entity that could do this (well, maybe Nokia could, but I'm doubting that one).Me to. Must be too obvious. I have limited free calls every month, and it would really help to know when I'm close to my limit. Maybe the wp-API won't allow it or something...
You are correct - there is no API that would allow an app to intercept a phone call, either incoming or outgoing (you'd have to intercept, let the app do something, then pass it back - all in a matter of milliseconds, of course). Microsoft is the only entity that could do this (well, maybe Nokia could, but I'm doubting that one).
Your carrier could do this. AT&T offers an app called myAT&T that tracks phone, data and SMS usage for all devices on an account. AT&T consumers can also get this information by triggering a message pressing #XXXX and send.