Pandora or slacker after a call...?

My guess is that both of them honor the call by initiating a pause, similar to when you pull out headphones, or remove AC power. Those acts don't resume once headphones are plugged in or AC power restored, you have to manually unpause. That's just a hunch on my part, but would be my first guess.

YouTube doesn't resume either. I can't remember if Safari streams and iTunes streams do. If they were able to do it Pandora sure would, so I doubt the APIs are available. Although, as I mentioned, even Apple doesn't use them much outside of the iPod.

Have you tried putting music on then RECEIVING a phone call?

My guess is that by receiving a phone call it will work as you expect. But because you are initiating a phone call you're effectively telling the phone to stop the music since only one sound source can be active at once so it's turning the music off.

By receiving a phone call you're agreeing to answer the call and it may pause it and then continue when the call is over.

But that may not work either (again, worth trying to see) because my understanding of the multi-tasking system for audio is that the program will be terminated if audio is not sent to the sound system continually. If there's a delay of a certain number of seconds the app is terminated. Which may be what's happening here. A long phone call may terminate the app and it won't start playing once the call is over.