CallBar works on any screen at anytime regardless of what you may be doing. It might be my favorite and most important tweak. It's up there with BiteSMS. Those 2 go hand in hand for the exact same reasons.
There are a few tweaks that can change the camera grabber to open any app you'd like. I remember trying to help someone else with this a few weeks ago. If I remember correctly, most of the tweaks (there are 3 I can think of right off the bat) weren't updated for iOS 6 yet. The person I was helping settled on using LockSliderz so they could add extra icons to their lockscreen to open whatever app they selected. You can use this to add the phone app to your lockscreen. You can have up to 8 different icons/apps open from the lockscreen. Hope this helps at least get you started. Good luck.