If you have no need for multi-tasking, then why would you need to leave the phone screen?
This phone and phone OS was designed to give you options. That's the allure behind Android.
What you want is perfectly feasible and perfectly reasonable--to you, and seemingly to you alone. Unfortunately, that means:
1) there's no native feature for your particular use case, and there probably never will be
2) nobody has made a custom ROM that does what you want, and probably no one ever will
It's perfectly reasonable to know what you want. However, you need to know where the rest of the world is, and you need to know that probably you're the minority. Your choices are to use what's readily available and adapt, or get another phone. Sorry, it's just the reality.
Edit: Oh yeah, what you could do is write your own custom ROM with a custom phone app that does exactly what you want. Or you could pay someone to do this. But I guess mentioning this is moot, because if you were willing to do this, you wouldn't be asking. But remember, Android allows you to do exactly what you want--you have the freedom to write your own software to do this. The limitation here isn't the phone or the software, but the fact that you have a pretty obscure request.